Right, that's us done for today. Keep an eye out for Andrew Benson's day one testing report going up on the website shortly.
Thanks for your company today and let's do it all again tomorrow.
Third pre-season test: Barcelona - day one times
1. Felipe Massa (Williams) 1:23.500 (103 laps)
2. Marcus Ericsson (Sauber) 1:24.276 (122)
3. Lewis Hamilton (Mercedes) 1:24.881 (48)
4. Daniil Kvyat (Red Bull) 1:25.947 (75)
5. Romain Grosjean (Lotus) 1:26.177 (75)
6. Kimi Raikkonen (Ferrari) 1:26.327 (80)
7. Carlos Sainz (Toro Rosso) 1:26.962 (86)
8. Jenson Button (McLaren) 1:31.479 (7)
Chequered flag
And that's us done for today.
The first day of the final pre-season test ends with Felipe Massa quickest for Williams, ahead of Sauber's Marcus Ericsson and the Mercedes of Lewis Hamilton.
We'll wrap up all the times for you in a moment.
Five minute warning
We're winding down in Barcelona. Ericsson, Kvyat, Grosjean, Raikkonen and Sainz are out on track.
Any late drama to come?
Ericsson up to P2
We sang Sauber's praises a short time ago and things are looking even better for the Swiss outfit now. Now shod with super-soft tyres, Marcus Ericsson's car has slotted into second after a 1:24.610.
Ok, it's probably on light fuel and is the equivalent of a goalkeeper making a save 'for the cameras', but what the hell.
Don't forget, this is a team that failed to score a point last season for the first time in their history. On this evidence a repeat of that misery in 2015 is unlikely.
Toro Rosso back on track
Toro Rosso on Twitter: "We lost a bit of track time this afternoon, but team has done a great job working against the clock & we're out again."
Carlos Sainz's Toro Rosso ground to a halt a couple of hours ago with an unspecified problem.
Williams are good - but Mercedes still have the edge
"Felipe Massa has set a new fastest time of the winter on Thursday with his pace-setting lap of one minute 23.500 seconds. But that is still not necessarily as impressive as the 1:24.881 Lewis Hamilton set in the morning.
"Let me explain. For one thing, Massa was on soft tyres and Hamilton on mediums. And the softs are reckoned to be 1.2secs a lap faster. Then there is the fact that Massa did only one flying lap, and Hamilton did his time on a 12-lap run.
"Assuming that each had only enough fuel for the run they did - a big assumption, admittedly - that means Hamilton could have gone about another 0.5secs quicker. Which would put him on a 1:23.0. And then bear in mind that Hamilton's team-mate Nico Rosberg did a 1:24.321 on mediums on Sunday."
Ericsson impresses for Sauber
It's been a good innings from Sauber today.
They started off the day with a lovely pic of the new C34 idling in a chilly pit lane (see 09:07), and have ended it with more than 100 laps in the bank and some strong times on the board to boot.
Marcus Ericsson has not put a wheel wrong today, ticking off 110 laps and counting. With a best time of 1:25.330, the Swede is currently third fastest, ahead of Red Bull, Lotus and Ferrari.
Getty ImagesCopyright: Getty Images
Problems at Mercedes
While nothing drastic, today's gremlins are a significant setback for the Mercedes team.
After minimal disruption to their pre-season running so far, to lose an entire afternoon's running at this stage is far from ideal, particularly with Williams hitting their straps.
It's is particularly tough on Lewis Hamilton, who has lost crucial running time and will now just have one more day in the car before the action proper kicks off in Australia in a few weeks.
Getty ImagesCopyright: Getty Images
Mercedes finished for the day
Mercedes on Twitter: "That's our day done. Problem diagnosed as MGUK failure. Crew working hard to make sure we're back in business tomorrow. #BackToWork #F1"
'MGUK' is the Motor Generator Unit - Kinetic. This is one of two energy recovery systems that comprise the ERS power-boost system.
By the way...
The thought belatedly occurs that Ron Dennis's "devoid of all injuries" comment earlier was a classic of the Ron-speak genre.
That is all.
Getty ImagesCopyright: Getty Images
Pictures of the day so far
Getty ImagesCopyright: Getty Images
Kimi Raikkonen has quietly got on with the task for Ferrari today. The 2007 world champion has churned out 54 laps for the Scuderia without ever really catching the eye. His best time is nearly three seconds off pacesetter Felipe Massa, albeit on slower tyres.
Pictures of the day so far
Toro RossoCopyright: Toro Rosso
Toro Rosso's Carlos Sainz blatted along happily for 59 laps today before a problem forced the STR10 to grind to a halt. He's yet to get back out with a little over an hour remaining.
Get involved - #bbcf1
Alec Byhurst: "The Williams pace is genuine, can't quite match Mercedes but they're a solid second. Australia always throws a strange result though."
carmic: "McLaren are still waiting to announce a title sponsor. Do you think we will see them in Australia in the yellow & black of the AA?"
Dan White: "I'm a little confused by McLaren Honda, haven't they had a whole year to get ready?"
James Docherty: "Continual coverage of McLaren 'testing woes' but Force India have 0 miles on their 2015 car. BBC F1 coverage currently a 1-note tune."
Michael Boxwell: "Great news that Manor Racing appear to be back. Can't wait to see their new livery, even if 2015 is a rebuilding year."
The times they are a-changin'
Here's where we stand with around 90 minutes left to run today.
Felipe Massa sits atop the pile with his soft-tyre-aided 1:23.500, having earlier set a time on the mediums that compared favourably to the hitherto dominant-looking Mercedes.
Special mention to Marcus Ericsson too. The Swede gets a biscuit and a pat on the head as today's endurance star, cracking a century of laps for Sauber.
1. Felipe Massa (Williams) 1:23.500 (77 laps)
2. Lewis Hamilton (Mercedes) 1:24.881 (48)
3. Daniil Kvyat (Red Bull) 1:25.947 (58)
4. Romain Grosjean (Lotus) 1:26.177 (46)
5. Kimi Raikkonen (Ferrari) 1:26.327 (45)
6. Carlos Sainz (Toro Rosso) 1:27.261 (59)
7. Marcus Ericsson (Sauber) 1:27.802 (100)
8. Jenson Button (McLaren) 1:31.479 (7)
Hybrid system problems for Mercedes
Lewis Hamilton's absence from the track has been explained anyway.
Mercedes have tweeted: "Failure in the Hybrid system keeping us in the garage currently. Crew working hard to fix it as we speak."
Getty ImagesCopyright: Getty Images
Get involved
We'd like your views on what we've seen so far today.
What do you make of Williams' pace - are they a genuine challenger to Mercedes? What now for McLaren after another difficult day? And what of Ferrari and Red Bull?
Any and all comments welcome. Send them in using #bbcf1
F1 journalist Dan Knutson in Barcelona
"A different type of F1 refuelling stop. The transporters have been here in Barcelona for days, so their diesel fuel tanks need to be refilled in order to run the generators that power the lights and auxiliary equipment."
KnutsonCopyright: Knutson
Massa improves further on soft tyre
After coming close to the best medium-tyre lap time of the two Barcelona tests thus far, it's no surprise to see Felipe Massa strap on a pair of soft boots and go for the glory of the fastest time of the test overall.
The Brazilian's time of 1:23.500 is half a second faster than Lotus' Roman Grosjean managed in the first Barcelona test, which had stood as the fastest overall time, although set on the super-soft tyre.
"Well, here's a surprise. Unexpectedly, Lotus have announced that Spanish female GP3 driver Carmen Jorda has joined the team as development driver.
"It is an echo of the situation at Williams with Susie Wolff, who had that title last year before being given an upgrade to test driver this season.
"Jorda will work in the simulator this year with the aim of readying herself for a test in the F1 car later this year."
LotusCopyright: Lotus
Ou est Lewis?
But where, I hear you asking, is Lewis Hamilton while Massa is out there thundering around?
Well, he remains in the pits, his lap tally still standing at 48, as it was at lunchtime.
There's no suggestion of any major drama in the Merc garage.
Massa goes quickest
He's been threatening it for a while now and finally he delivers. Felipe Massa sets the quickest time of the day with a 1:24.687, around two-tenths up on Lewis Hamilton's previous best.
Crucially, that lap from the Williams man came on the medium compound tyre that Hamilton was using this morning, and is just three-tenths off the best medium-tyre time of the two Barcelona tests so far, set by Hamilton's Mercedes team-mate Nico Rosberg.
Of course the usual rider about fuel levels applies, but that is nevertheless encouraging for everyone who does not want to see the Mercedes run and hide come Melbourne.
F1 journalist Dan Knutson in Barcelona
"While Felipe Massa is out there testing the new aero package on the Williams, his team-mate Valtteri Bottas is monitoring things from the pit wall."
KnutsonCopyright: Knutson
Track clear. Green running again
Sainz's car has been cleared and we're under way again. Just over two and a half hours of this first day remains.
Red flag
Carlos Sainz of Toro Rosso has stopped out on track and we have our second red flag of the day.
The Spanish rookie has stopped at turn 10 but there's no word on the nature of the problem yet.
Sainz has run well to this point, amassing 59 laps with a best time of 1:27.261.
"Not long after Ron Dennis's half-hour news conference on the subject of Fernando Alonso's wellbeing, the team have suffered yet another blow.
"They say they are stopping running after discovering a water leak in the engine. Jenson Button had done just seven laps so far.
"If McLaren and Honda can't fix these repeated problems, its getting dangerously close to the point where even if Alonso is fit for Australia, you'd have to question whether there is any point him turning up."
Getty ImagesCopyright: Getty Images
Massa chipping into Hamilton's time
Meanwhile, back on track, Felipe Massa is continuing to pound round. He's now improved to a 1:25.133 - just a quarter of a second from Lewis Hamilton's best time.
McLaren turn it in for the day
McLaren on Twitter: "We're bringing the shutters down on today - a hydraulic leak means we need to change the engine. #TestingIsTesting"
McLaren's day ends after just seven laps, the best of which was more than six seconds shy of the benchmark pace of Mercedes' Lewis Hamilton. McLaren's troubled pre-season continues...
Hamilton's cockpit selfie
Lewis Hamilton is much loved at BBC Towers by those who cobble together our esteemed F1 Gossip page because the guy is social media gold.
When he's not posting a picture of himself sitting on the wing of his private jet, he's eyeballing Taylor Swift at the Brits - or, as below, taking a selfie in the cockpit of his Mercedes.
Keep up the good work Lewis. The Gossip column needs you.
Lewis Hamilton/TwitterCopyright: Lewis Hamilton/Twitter
Massa pumping the laps in
After a brief lull, it's all go on track now. Felipe Massa has been busy since the resumption. He's now done more laps than anyone today - 52 as I type - with Williams seemingly switching to a race simulation for this afternoon.
Carlos Sainz is up to sixth place in the Toro Rosso thanks to a 1:27.640, while the Ferrari of Kimi Raikkonen has also improved. The Finn has done a 1:26.553 on the medium tyre and is up to fourth.
Pictures of the day so far
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Red Bull's Daniil Kvyat has provided the day's (minor) drama thus far, stalling his RB11 on his way out the pits to bring out the red flag inside 10 minutes.
Pictures of the day so far
Getty ImagesCopyright: Getty Images
This has been a rare sight today - a McLaren rolling out of the garage. Jenson Button has managed just seven laps so far as the team continue to try to get to grips with their new challenger.
Track action remains at a premium
Felipe Massa's early outing after lunch came close to his morning best - punching in a 1:25.963 - but with that he's back in the pits.
Ferrari's Kimi Raikkonen and Carlos Sainz of Toro Rosso also ventured out but also returned to the pit lane in quick order.
Still we wait for this day to catch light...
Who, what, when, where? The answers
Thanks to all of you who played our blurry-picture game thing earlier (the title needs some work).
Among incorrect driver guesses were Ayrton Senna, Alain Prost, Allan McNish, Keke Rosberg and David Coulthard.
For the year, 1986, 1988, 1989, 1990, 1993 and 1996 were all wrong.
Erroneous track guesses included Adelaide, Silverstone, Estoril and Imola.
But plenty of you were right, and you told me it was Mika Hakkinen and his stricken McLaren, just moments after he had triggered a huge multi-car crash at the start of the 1994 German Grand Prix at Hockenheim.
Getty ImagesCopyright: Getty Images
Green flag. We're back testing
Ok, lunch time over. Time for more track action.
The rain has stopped and Felipe Massa of Williams is the first man out.
Let's do this.
Who, what, when, where?
Time for more idle nonsense to pass the lunch break.
We have (very scientifically) blurred the below picture.
If you can work out from this who the driver and car is, what just happened and at which track and in which year it occurred, you will earn my lasting respect.
Answer using #bbcf1
Getty ImagesCopyright: Getty Images
Alonso set to race in Australia - Dennis
EPACopyright: EPA
Andrew Benson and Lee McKenzie's story from today's McLaren news conference is now online.
McLaren chairman Ron Dennis said Fernando Alonso is "completely lucid", "normal" and should be fine to race in the season-opening Australian Grand Prix despite last Sunday's crash.
With the lunch break comes a chance to catch up on our caption competition we set you earlier. Here's a few of your responses to the below pic of Max and Jos Verstappen.
Nick Peart: "If you want to look old enough to drive in F1 I think we should get you a cap that fits... What about one of these?"
lfcrule1972: "Look Max, there's a new game on the App Store called FlappyMcLaren. Seems there are a lot of bugs in it though."
Richard Chappell "Look at this Max, Madonna was on stage and a Minotaur pulled her over by a cloak. Best Brit Awards ever don't you think?"
Chris Cullen: "So son, you just swipe right if you fancy her and left if you don't. Simple."
Getty ImagesCopyright: Getty Images
And that's lunch
That's the morning session done. Here's the times and laps competed so far.
Mercedes' Lewis Hamilton leads the way in both lap time and mileage. At other end, the McLaren of Jenson Button is well off the pace after managing just seven laps.
1. Lewis Hamilton (Mercedes) 1:24.881 (48 laps)
2. Felipe Massa (Williams) 1:25.763 (29)
3. Daniil Kvyat (Red Bull) 1:25.947 (45)
4. Romain Grosjean (Lotus) 1:27.079 (33)
5. Marcus Ericsson (Sauber) 1:27.802 (33)
6. Kimi Raikkonen (Ferrari) 1:27.813 (28)
7. Carlos Sainz (Toro Rosso) 1:27.866 (34)
8. Jenson Button (McLaren) 1:31.479 (7)
All quiet on track
Back on the asphalt, there's little to report.
That rain shower rather took the life out of what was already a low-key session. Lewis Hamilton and Daniil Kvyat went out in the Mercedes and Red Bull respectively, but their times were predictably well off the pace.
"McLaren chairman Ron Dennis has said at a news conference that Fernando Alonso was unconscious for 'just a few seconds' after his crash on Sunday.
"Dennis says Alonso has got no injury. He should be fine to race in Australia and in normal circumstances he'll be racing. This is recovery time.
"I asked Dennis what Alonso remembers of the accident, and he said they had not had that conversation yet. He says every input by Alonso in car was normal up until the crash. The radio stayed on and they heard breathing but no contact."
McLaren news conference
McLaren chairman Ron Dennis has, as expected, been talking about Fernando Alonso's crash last week.
Dennis has told assembled journalists in the Barcelona paddock that Alonso was briefly knocked out in the accident but expects the Spaniard to race in Australia, pending clearance from doctors.
"McLaren chairman Ron Dennis and racing director Eric Boullier are holding a news conference, due to start imminently.
"This has been called this morning and the pair will undoubtedly be asked about Fernando Alonso's condition. There are many unanswered questions surrounding Alonso's accident and his medical wellbeing.
"High among them is one arising from a report in Italy's Gazzetta Dello Sport this morning, which stated that Alonso was suffering in hospital from 'retrograde amnesia', which not only affected his recollection of the accident, but also 'other aspects of his life'.
"There is no confirmation whether this is true or not, but Dennis will inevitably be asked to confirm whether that is the case."
Spoke too soon
And with that, Red Bull's Daniil Kvyat fires up his Renault engine and heads out.
We're back on track.
All is safely gathered in
Still no action on track at present. The rain shower is keeping all cars and drivers indoors for now.
Psssst! Fancy a gossip?
Want to see Lewis Hamilton look suitably impressed to be within a nose cone's distance of Taylor Swift? Of course you do.
"With its new car on the way to Spain, Force India has abandoned the 2014 VJM07 that ran during the four days of testing here last week."
KnutsonCopyright: Knutson
Get involved
Chris, Earl Ballard: "Expecting Mercedes to not just beat, but to absolutely demolish all comers come Australia qualifying."
Strodey123: "Not looking good for McLaren, but they're in the same position as Red Bull were a year ago, and they still won 3 races."
Stephen: "I know it's testing but I was worried about Williams' pace. Nice to see them speed up!"
F1 journalist Dan Knutson in Barcelona
"There were patches of blue sky this morning but now it is overcast. Some rain sprinkles started to fall at 11:45 a.m. local time."
All the cars are now in the pits as they wait for the shower to pass.
Massa up to second
Felipe Massa's stint on the medium tyre is going swimmingly.
The affable Brazilian has hauled his Williams up to second place with a 1:25.763, albeit still nearly a second off Hamilton's benchmark.
Getty ImagesCopyright: Getty Images
Caption competition
Carlos Sainz's stint in the car today has left Toro Rosso team-mate Max Verstappen to cool his heels in the pit lane.
It's given the 17-year-old a little father-son time with dad Jos, as this picture shows.
And it looks ripe for a bit of caption speculation if you ask me. Send your suggestions on Twitter using #bbcf1.
Getty ImagesCopyright: Getty Images
Sainz finally sets a time
Carlos Sainz has certainly done the development miles in the Toro Rosso today.
The rookie racked up more than 20 laps before he set a time - the legacy of a series of practice pit stops.
The Spaniard's went and done a 1:27.866, dropping Jenson Button's McLaren to the foot of the standings.
Medium tyre klaxon
In a rare display of unity, all the drivers are currently on medium tyres, which makes interpreting their lap times a tad easier - although the tickly issue of fuel levels of course still fogs the issue.
Still, it's something.
Now on the mediums, Williams' Felipe Massa picks up three-tenths on his previous best as he posts a 1:26.083 - a whisper behind Red Bull's Daniil Kvyat, who himself has improved to 1:25.947. Kimi Rakkonen's Ferrari is also donning medium boots and he's up to 1:27.813.
Get involved
Don't forget that throughout pre-season testing you can get in touch with us here at BBC Sport to have your say on the action.
Tag your messages #bbcf1 and we'll publish as many as we can.
"It's not always easy to construct a picture of comparative form from pre-season testing, but an image is starting to emerge. It seems clear from last week's Barcelona test particularly that Mercedes still have a significant advantage over their rivals at the front of the field.
"The team themselves are playing this down, pointing out that fuel loads are unknown, different tyres have been used by different teams and most of all that everyone has upgrades for this final test and some yet more for the first race.
"But most insiders seem to agree that Mercedes are a step ahead - how much is not clear, but some believe it is as much as a second a lap - and that the next bunch of Red Bull, Ferrari and Williams are closely matched.
"Will the next four days consolidate or change those impressions?"
How they stand
Well, it's fair to say no one's mind has been blow by the action on the track thus far. But this is F1 and there's always a good tale around the corner...
For now, here's a look at the times as things stand two and a quarter hours into this ninth day of pre-season testing.
1. Lewis Hamilton (Mercedes) 1:24.881 (29 laps)
2. Daniil Kvyat (Red Bull) 1:25.987 (26)
3. Felipe Massa (Williams) 1:26.399 (19)
4. Romain Grosjean (Lotus) 1:27.525 (24)
5. Marcus Ericsson (Sauber) 1:27.802 (27)
6. Kimi Raikkonen (Ferrari) 1:29.590 (18)
7. Jenson Button (McLaren) 1:31.479 (7)
8. Carlos Sainz (Toro Rosso) no time (24)
De Ferran warning for McLaren and Alonso
Getty ImagesCopyright: Getty Images
Here's an interesting tale from our website last night that you may have missed.
The recuperating Fernando Alonso has been warned not to rush back to racing following the crash that left him concussed and forced him to miss this test.
Gil De Ferran, a winner of the Indianapolis 500, two-time IndyCar champion and a man who has suffered more than his fair share of concussions, told BBC Sport that returning too soon is "the worst thing a driver can do".
"While the thoughts and best wishes of everyone are with Fernando Alonso as he recuperates at his home in Oviedo, 560 miles away, following his crash last Sunday, his McLaren team will inevitably remain a major focus of attention at the Circuit De Barcelona-Catalunya these next four days.
"Quite apart from missing their star driver, McLaren are so far in no sort of shape for the new season, after eight severely troubled days of testing.
"Last week's test was ruined by continuing faults with a relatively minor part - a seal on the motor generator unit recovering energy from the rear axle. A fourth version of this part is to be tried this week in an attempt to give the team some proper running at last."
Button on the board
Jenson Button has finally posted a lap time in the McLaren Honda, but his 1:31.479 - some 6.5secs down on Hamilton - suggests the team is still finding its legs on this first morning.
McLaren chairman Ron Dennis has been pictured in the back of the team garage. There's a man with plenty to think about.
New Force India en route to Barcelona
Force India have endured a troubled pre-season programme, skipping the first test in Jerez and then running a modified 2014 car in the second.
However, the team said this week that they would have their new challenger on track this Friday - and a tweet this morning has confirmed the new motor is boxed up and on its way from Silverstone to Spain.
"Our VJM08 is currently on its way to Barcelona; we'll be aiming for action on Friday afternoon and on the two full final days," the team said.
The team added that after the car is "completed on Friday morning", Nico Hulkenberg will be first behind the wheel. The German and Sergio Perez will share driving duties across the three days.
"All teams will have upgrades at this test, with a view to preparations for the first race. And Toro Rosso's is the first to break cover.
"The team have a new front-nose design, replacing the smooth, rounded nose design of before with a Williams/Red Bull-Style stump. The aim is to improve airflow under the car - and thereby increase downforce - while satisfying the new safety regulations."
Getty ImagesCopyright: Getty Images
"I'm where I love being"
As you're reading this blog there's every chance you dream - or once dreamt - of being an F1 driver.
So it's good to know Lotus' Romain Grosjean is fully appreciative of where he is today.
"Wishing you all a very good day!" he wrote on Twitter. "I'm where I love being, behind the wheel..."
Lotus F1Copyright: Lotus F1
Hamilton into the 1:24s
Meanwhile, that ominous-looking Mercedes is still pounding away. Hamilton improves to 1:24.881 on the medium tyre. That's around five-tenths off the pace team-mate Nico Rosberg managed on the same tyre on the final day of the second test at the same circuit.
Picture perfect Sauber
The Sauber team have posted this rather lovely picture on Twitter of Marcus Ericsson sitting patiently at the end of the Barcelona pit lane waiting to light up his tyres with a practice start.
As you can see, it's a perfect day for a good hoon around the Circuit de Catalunya.
SauberCopyright: Sauber
Low-key start for Button and Honda
After a trouble pre-season testing programme so far and driver Fernando Alonso sidelined, there's plenty of interest in the progress of the new McLaren Honda car.
Jenson Button, complete with new Niki Lauda-esque helmet, is the man behind the wheel today and it's been a low-key start so far - just two installation laps from the Briton and no time on the charts as yet.
Getty ImagesCopyright: Getty Images
Mercedes the benchmark - Horner
With Hamilton top of the pile now seems as good as time as any to bring you a line from Red Bull boss Christian Horner, who believes Mercedes are still the team to beat in Formula 1, describing them as "the benchmark".
Horner was speaking on a special F1 edition of A Question of Sport broadcast last night.
If you missed last night's show, it will be broadcast again on Friday, at 19:30 GMT on BBC One.
BBCCopyright: BBC
Hamilton tops early timesheets
Lewis Hamilton has set the early pace inside the first hour, the Mercedes driver quickest in a time of 1:26.437 on the medium tyre, around four-tenths up on Williams' Felipe Massa, who is on the hard.
New package for Toro Rosso
Toro Rossotweet: "Carlos is back in the garage.. Spot the changes on our car. We're running a new package #GOTOROROSSO"
Toro RossoCopyright: Toro Rosso
Early pace setter
Kimi Raikkonen's settled in to a few decent laps so far.
He opens up with a 1:32.451 and immediately wipes it off with a 1:30.7, a 1:30.692, a 1:30.271 and a 1:30.526.
Restart
Getty ImagesCopyright: Getty Images
Kvyat's RB11 is pushed to the garage after a nightmare start for the Russian. Green flags are out and Lewis Hamilton joins Raikkonen out on the track.
Red flag
Not a great start to the day for Kvyat. He sets off for an installation lap and only gets as far as the end of the pit lane before grinding to a halt.
Out comes the red flag.
Today's likely lads
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So, as well as Kimi Raikkonen, who as Lee McKenzie said has been first out on the track this morning, we're expecting to see seven other drivers go through their paces today.
Fresh from his red carpet appearance at last night's Brit Awards, world champion Lewis Hamilton will be heading out in his Mercedes, Jenson Button will put the McLaren through its paces, Marcus Ericsson will test out the Sauber, Romain Grosjean his Lotus, Daniil Kvyat in the Red Bull, Felipe Massa for Williams and Carlos Sainz in the Toro Rosso are all set to head out today.
"There is much to dwell upon as the final pre-season test of 2015 gets under way.
"Two weeks before the F1 circus sets up camp in Melbourne's Albert Park, teams will be introducing first-race upgrades to their cars these four days in Barcelona, and trying to tick all the boxes in terms of preparation.
"There will even be a new car to look at in the shape of the 2015 Force India.
"But before the cars are shipped off to Australia one question hangs above all others - will Fernando Alonso, arguably the sport's greatest participant, even be there?"
On Twitter: It's cold in Barcelona at the final #f1 preseason test and it's Raikkonen who is the first car out on track
They're off.....again
After the briefest of breaks, the teams and drivers are back in Barcelona.
It's the final four days of pre-season testing before the big off in Melbourne on 15 March.
With preparations for the season opener at Albert Park ramping up by the day, and crucial mileage still to be accumulated, we'll bring you all the latest twists and turns from the track.
Live Reporting
Jamie Strickland
All times stated are UK
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Latest PostGoodbye for now
Right, that's us done for today. Keep an eye out for Andrew Benson's day one testing report going up on the website shortly.
Thanks for your company today and let's do it all again tomorrow.
Third pre-season test: Barcelona - day one times
1. Felipe Massa (Williams) 1:23.500 (103 laps)
2. Marcus Ericsson (Sauber) 1:24.276 (122)
3. Lewis Hamilton (Mercedes) 1:24.881 (48)
4. Daniil Kvyat (Red Bull) 1:25.947 (75)
5. Romain Grosjean (Lotus) 1:26.177 (75)
6. Kimi Raikkonen (Ferrari) 1:26.327 (80)
7. Carlos Sainz (Toro Rosso) 1:26.962 (86)
8. Jenson Button (McLaren) 1:31.479 (7)
Chequered flag
And that's us done for today.
The first day of the final pre-season test ends with Felipe Massa quickest for Williams, ahead of Sauber's Marcus Ericsson and the Mercedes of Lewis Hamilton.
We'll wrap up all the times for you in a moment.
Five minute warning
We're winding down in Barcelona. Ericsson, Kvyat, Grosjean, Raikkonen and Sainz are out on track.
Any late drama to come?
Ericsson up to P2
We sang Sauber's praises a short time ago and things are looking even better for the Swiss outfit now. Now shod with super-soft tyres, Marcus Ericsson's car has slotted into second after a 1:24.610.
Ok, it's probably on light fuel and is the equivalent of a goalkeeper making a save 'for the cameras', but what the hell.
Don't forget, this is a team that failed to score a point last season for the first time in their history. On this evidence a repeat of that misery in 2015 is unlikely.
Toro Rosso back on track
Toro Rosso on Twitter: "We lost a bit of track time this afternoon, but team has done a great job working against the clock & we're out again."
Carlos Sainz's Toro Rosso ground to a halt a couple of hours ago with an unspecified problem.
Williams are good - but Mercedes still have the edge
Andrew Benson
Chief F1 writer
"Felipe Massa has set a new fastest time of the winter on Thursday with his pace-setting lap of one minute 23.500 seconds. But that is still not necessarily as impressive as the 1:24.881 Lewis Hamilton set in the morning.
"Let me explain. For one thing, Massa was on soft tyres and Hamilton on mediums. And the softs are reckoned to be 1.2secs a lap faster. Then there is the fact that Massa did only one flying lap, and Hamilton did his time on a 12-lap run.
"Assuming that each had only enough fuel for the run they did - a big assumption, admittedly - that means Hamilton could have gone about another 0.5secs quicker. Which would put him on a 1:23.0. And then bear in mind that Hamilton's team-mate Nico Rosberg did a 1:24.321 on mediums on Sunday."
Ericsson impresses for Sauber
It's been a good innings from Sauber today.
They started off the day with a lovely pic of the new C34 idling in a chilly pit lane (see 09:07), and have ended it with more than 100 laps in the bank and some strong times on the board to boot.
Marcus Ericsson has not put a wheel wrong today, ticking off 110 laps and counting. With a best time of 1:25.330, the Swede is currently third fastest, ahead of Red Bull, Lotus and Ferrari.
Problems at Mercedes
While nothing drastic, today's gremlins are a significant setback for the Mercedes team.
After minimal disruption to their pre-season running so far, to lose an entire afternoon's running at this stage is far from ideal, particularly with Williams hitting their straps.
It's is particularly tough on Lewis Hamilton, who has lost crucial running time and will now just have one more day in the car before the action proper kicks off in Australia in a few weeks.
Mercedes finished for the day
Mercedes on Twitter: "That's our day done. Problem diagnosed as MGUK failure. Crew working hard to make sure we're back in business tomorrow. #BackToWork #F1"
'MGUK' is the Motor Generator Unit - Kinetic. This is one of two energy recovery systems that comprise the ERS power-boost system.
By the way...
The thought belatedly occurs that Ron Dennis's "devoid of all injuries" comment earlier was a classic of the Ron-speak genre.
That is all.
Pictures of the day so far
Kimi Raikkonen has quietly got on with the task for Ferrari today. The 2007 world champion has churned out 54 laps for the Scuderia without ever really catching the eye. His best time is nearly three seconds off pacesetter Felipe Massa, albeit on slower tyres.
Pictures of the day so far
Toro Rosso's Carlos Sainz blatted along happily for 59 laps today before a problem forced the STR10 to grind to a halt. He's yet to get back out with a little over an hour remaining.
Get involved - #bbcf1
Alec Byhurst: "The Williams pace is genuine, can't quite match Mercedes but they're a solid second. Australia always throws a strange result though."
carmic: "McLaren are still waiting to announce a title sponsor. Do you think we will see them in Australia in the yellow & black of the AA?"
Dan White: "I'm a little confused by McLaren Honda, haven't they had a whole year to get ready?"
James Docherty: "Continual coverage of McLaren 'testing woes' but Force India have 0 miles on their 2015 car. BBC F1 coverage currently a 1-note tune."
Michael Boxwell: "Great news that Manor Racing appear to be back. Can't wait to see their new livery, even if 2015 is a rebuilding year."
The times they are a-changin'
Here's where we stand with around 90 minutes left to run today.
Felipe Massa sits atop the pile with his soft-tyre-aided 1:23.500, having earlier set a time on the mediums that compared favourably to the hitherto dominant-looking Mercedes.
Special mention to Marcus Ericsson too. The Swede gets a biscuit and a pat on the head as today's endurance star, cracking a century of laps for Sauber.
1. Felipe Massa (Williams) 1:23.500 (77 laps)
2. Lewis Hamilton (Mercedes) 1:24.881 (48)
3. Daniil Kvyat (Red Bull) 1:25.947 (58)
4. Romain Grosjean (Lotus) 1:26.177 (46)
5. Kimi Raikkonen (Ferrari) 1:26.327 (45)
6. Carlos Sainz (Toro Rosso) 1:27.261 (59)
7. Marcus Ericsson (Sauber) 1:27.802 (100)
8. Jenson Button (McLaren) 1:31.479 (7)
Hybrid system problems for Mercedes
Lewis Hamilton's absence from the track has been explained anyway.
Mercedes have tweeted: "Failure in the Hybrid system keeping us in the garage currently. Crew working hard to fix it as we speak."
Get involved
We'd like your views on what we've seen so far today.
What do you make of Williams' pace - are they a genuine challenger to Mercedes? What now for McLaren after another difficult day? And what of Ferrari and Red Bull?
Any and all comments welcome. Send them in using #bbcf1
F1 journalist Dan Knutson in Barcelona
"A different type of F1 refuelling stop. The transporters have been here in Barcelona for days, so their diesel fuel tanks need to be refilled in order to run the generators that power the lights and auxiliary equipment."
Massa improves further on soft tyre
After coming close to the best medium-tyre lap time of the two Barcelona tests thus far, it's no surprise to see Felipe Massa strap on a pair of soft boots and go for the glory of the fastest time of the test overall.
The Brazilian's time of 1:23.500 is half a second faster than Lotus' Roman Grosjean managed in the first Barcelona test, which had stood as the fastest overall time, although set on the super-soft tyre.
Lotus sign Jorda as development driver
Andrew Benson
Chief F1 writer
"Well, here's a surprise. Unexpectedly, Lotus have announced that Spanish female GP3 driver Carmen Jorda has joined the team as development driver.
"It is an echo of the situation at Williams with Susie Wolff, who had that title last year before being given an upgrade to test driver this season.
"Jorda will work in the simulator this year with the aim of readying herself for a test in the F1 car later this year."
Ou est Lewis?
But where, I hear you asking, is Lewis Hamilton while Massa is out there thundering around?
Well, he remains in the pits, his lap tally still standing at 48, as it was at lunchtime.
There's no suggestion of any major drama in the Merc garage.
Massa goes quickest
He's been threatening it for a while now and finally he delivers. Felipe Massa sets the quickest time of the day with a 1:24.687, around two-tenths up on Lewis Hamilton's previous best.
Crucially, that lap from the Williams man came on the medium compound tyre that Hamilton was using this morning, and is just three-tenths off the best medium-tyre time of the two Barcelona tests so far, set by Hamilton's Mercedes team-mate Nico Rosberg.
Of course the usual rider about fuel levels applies, but that is nevertheless encouraging for everyone who does not want to see the Mercedes run and hide come Melbourne.
F1 journalist Dan Knutson in Barcelona
"While Felipe Massa is out there testing the new aero package on the Williams, his team-mate Valtteri Bottas is monitoring things from the pit wall."
Track clear. Green running again
Sainz's car has been cleared and we're under way again. Just over two and a half hours of this first day remains.
Red flag
Carlos Sainz of Toro Rosso has stopped out on track and we have our second red flag of the day.
The Spanish rookie has stopped at turn 10 but there's no word on the nature of the problem yet.
Sainz has run well to this point, amassing 59 laps with a best time of 1:27.261.
Hydraulic leak compounds problems at McLaren
Andrew Benson
Chief F1 writer
"Not long after Ron Dennis's half-hour news conference on the subject of Fernando Alonso's wellbeing, the team have suffered yet another blow.
"They say they are stopping running after discovering a water leak in the engine. Jenson Button had done just seven laps so far.
"If McLaren and Honda can't fix these repeated problems, its getting dangerously close to the point where even if Alonso is fit for Australia, you'd have to question whether there is any point him turning up."
Massa chipping into Hamilton's time
Meanwhile, back on track, Felipe Massa is continuing to pound round. He's now improved to a 1:25.133 - just a quarter of a second from Lewis Hamilton's best time.
McLaren turn it in for the day
McLaren on Twitter: "We're bringing the shutters down on today - a hydraulic leak means we need to change the engine. #TestingIsTesting"
McLaren's day ends after just seven laps, the best of which was more than six seconds shy of the benchmark pace of Mercedes' Lewis Hamilton. McLaren's troubled pre-season continues...
Hamilton's cockpit selfie
Lewis Hamilton is much loved at BBC Towers by those who cobble together our esteemed F1 Gossip page because the guy is social media gold.
When he's not posting a picture of himself sitting on the wing of his private jet, he's eyeballing Taylor Swift at the Brits - or, as below, taking a selfie in the cockpit of his Mercedes.
Keep up the good work Lewis. The Gossip column needs you.
Massa pumping the laps in
After a brief lull, it's all go on track now. Felipe Massa has been busy since the resumption. He's now done more laps than anyone today - 52 as I type - with Williams seemingly switching to a race simulation for this afternoon.
Carlos Sainz is up to sixth place in the Toro Rosso thanks to a 1:27.640, while the Ferrari of Kimi Raikkonen has also improved. The Finn has done a 1:26.553 on the medium tyre and is up to fourth.
Pictures of the day so far
Red Bull's Daniil Kvyat has provided the day's (minor) drama thus far, stalling his RB11 on his way out the pits to bring out the red flag inside 10 minutes.
Pictures of the day so far
This has been a rare sight today - a McLaren rolling out of the garage. Jenson Button has managed just seven laps so far as the team continue to try to get to grips with their new challenger.
Track action remains at a premium
Felipe Massa's early outing after lunch came close to his morning best - punching in a 1:25.963 - but with that he's back in the pits.
Ferrari's Kimi Raikkonen and Carlos Sainz of Toro Rosso also ventured out but also returned to the pit lane in quick order.
Still we wait for this day to catch light...
Who, what, when, where? The answers
Thanks to all of you who played our blurry-picture game thing earlier (the title needs some work).
Among incorrect driver guesses were Ayrton Senna, Alain Prost, Allan McNish, Keke Rosberg and David Coulthard.
For the year, 1986, 1988, 1989, 1990, 1993 and 1996 were all wrong.
Erroneous track guesses included Adelaide, Silverstone, Estoril and Imola.
But plenty of you were right, and you told me it was Mika Hakkinen and his stricken McLaren, just moments after he had triggered a huge multi-car crash at the start of the 1994 German Grand Prix at Hockenheim.
Green flag. We're back testing
Ok, lunch time over. Time for more track action.
The rain has stopped and Felipe Massa of Williams is the first man out.
Let's do this.
Who, what, when, where?
Time for more idle nonsense to pass the lunch break.
We have (very scientifically) blurred the below picture.
If you can work out from this who the driver and car is, what just happened and at which track and in which year it occurred, you will earn my lasting respect.
Answer using #bbcf1
Alonso set to race in Australia - Dennis
Andrew Benson and Lee McKenzie's story from today's McLaren news conference is now online.
McLaren chairman Ron Dennis said Fernando Alonso is "completely lucid", "normal" and should be fine to race in the season-opening Australian Grand Prix despite last Sunday's crash.
Read the full article here.
Get involved - caption competition. #bbcf1
With the lunch break comes a chance to catch up on our caption competition we set you earlier. Here's a few of your responses to the below pic of Max and Jos Verstappen.
Nick Peart: "If you want to look old enough to drive in F1 I think we should get you a cap that fits... What about one of these?"
lfcrule1972: "Look Max, there's a new game on the App Store called FlappyMcLaren. Seems there are a lot of bugs in it though."
Richard Chappell "Look at this Max, Madonna was on stage and a Minotaur pulled her over by a cloak. Best Brit Awards ever don't you think?"
Chris Cullen: "So son, you just swipe right if you fancy her and left if you don't. Simple."
And that's lunch
That's the morning session done. Here's the times and laps competed so far.
Mercedes' Lewis Hamilton leads the way in both lap time and mileage. At other end, the McLaren of Jenson Button is well off the pace after managing just seven laps.
1. Lewis Hamilton (Mercedes) 1:24.881 (48 laps)
2. Felipe Massa (Williams) 1:25.763 (29)
3. Daniil Kvyat (Red Bull) 1:25.947 (45)
4. Romain Grosjean (Lotus) 1:27.079 (33)
5. Marcus Ericsson (Sauber) 1:27.802 (33)
6. Kimi Raikkonen (Ferrari) 1:27.813 (28)
7. Carlos Sainz (Toro Rosso) 1:27.866 (34)
8. Jenson Button (McLaren) 1:31.479 (7)
All quiet on track
Back on the asphalt, there's little to report.
That rain shower rather took the life out of what was already a low-key session. Lewis Hamilton and Daniil Kvyat went out in the Mercedes and Red Bull respectively, but their times were predictably well off the pace.
The lunch break looms.
McLaren news conference
Lee McKenzie
BBC F1 pit-lane reporter
"McLaren chairman Ron Dennis has said at a news conference that Fernando Alonso was unconscious for 'just a few seconds' after his crash on Sunday.
"Dennis says Alonso has got no injury. He should be fine to race in Australia and in normal circumstances he'll be racing. This is recovery time.
"I asked Dennis what Alonso remembers of the accident, and he said they had not had that conversation yet. He says every input by Alonso in car was normal up until the crash. The radio stayed on and they heard breathing but no contact."
McLaren news conference
McLaren chairman Ron Dennis has, as expected, been talking about Fernando Alonso's crash last week.
Dennis has told assembled journalists in the Barcelona paddock that Alonso was briefly knocked out in the accident but expects the Spaniard to race in Australia, pending clearance from doctors.
We'll have the full quotes for you very soon.
McLaren call unexpected news conference
Andrew Benson
Chief F1 writer
"McLaren chairman Ron Dennis and racing director Eric Boullier are holding a news conference, due to start imminently.
"This has been called this morning and the pair will undoubtedly be asked about Fernando Alonso's condition. There are many unanswered questions surrounding Alonso's accident and his medical wellbeing.
"High among them is one arising from a report in Italy's Gazzetta Dello Sport this morning, which stated that Alonso was suffering in hospital from 'retrograde amnesia', which not only affected his recollection of the accident, but also 'other aspects of his life'.
"There is no confirmation whether this is true or not, but Dennis will inevitably be asked to confirm whether that is the case."
Spoke too soon
And with that, Red Bull's Daniil Kvyat fires up his Renault engine and heads out.
We're back on track.
All is safely gathered in
Still no action on track at present. The rain shower is keeping all cars and drivers indoors for now.
Psssst! Fancy a gossip?
Want to see Lewis Hamilton look suitably impressed to be within a nose cone's distance of Taylor Swift? Of course you do.
That and other chat can be found on our world-famous F1 Gossip page.
F1 journalist Dan Knutson in Barcelona
"With its new car on the way to Spain, Force India has abandoned the 2014 VJM07 that ran during the four days of testing here last week."
Get involved
Chris, Earl Ballard: "Expecting Mercedes to not just beat, but to absolutely demolish all comers come Australia qualifying."
Strodey123: "Not looking good for McLaren, but they're in the same position as Red Bull were a year ago, and they still won 3 races."
Stephen: "I know it's testing but I was worried about Williams' pace. Nice to see them speed up!"
F1 journalist Dan Knutson in Barcelona
"There were patches of blue sky this morning but now it is overcast. Some rain sprinkles started to fall at 11:45 a.m. local time."
All the cars are now in the pits as they wait for the shower to pass.
Massa up to second
Felipe Massa's stint on the medium tyre is going swimmingly.
The affable Brazilian has hauled his Williams up to second place with a 1:25.763, albeit still nearly a second off Hamilton's benchmark.
Caption competition
Carlos Sainz's stint in the car today has left Toro Rosso team-mate Max Verstappen to cool his heels in the pit lane.
It's given the 17-year-old a little father-son time with dad Jos, as this picture shows.
And it looks ripe for a bit of caption speculation if you ask me. Send your suggestions on Twitter using #bbcf1.
Sainz finally sets a time
Carlos Sainz has certainly done the development miles in the Toro Rosso today.
The rookie racked up more than 20 laps before he set a time - the legacy of a series of practice pit stops.
The Spaniard's went and done a 1:27.866, dropping Jenson Button's McLaren to the foot of the standings.
Medium tyre klaxon
In a rare display of unity, all the drivers are currently on medium tyres, which makes interpreting their lap times a tad easier - although the tickly issue of fuel levels of course still fogs the issue.
Still, it's something.
Now on the mediums, Williams' Felipe Massa picks up three-tenths on his previous best as he posts a 1:26.083 - a whisper behind Red Bull's Daniil Kvyat, who himself has improved to 1:25.947. Kimi Rakkonen's Ferrari is also donning medium boots and he's up to 1:27.813.
Get involved
Don't forget that throughout pre-season testing you can get in touch with us here at BBC Sport to have your say on the action.
Tag your messages #bbcf1 and we'll publish as many as we can.
Post update
Andrew Benson
Chief F1 writer
"It's not always easy to construct a picture of comparative form from pre-season testing, but an image is starting to emerge. It seems clear from last week's Barcelona test particularly that Mercedes still have a significant advantage over their rivals at the front of the field.
"The team themselves are playing this down, pointing out that fuel loads are unknown, different tyres have been used by different teams and most of all that everyone has upgrades for this final test and some yet more for the first race.
"But most insiders seem to agree that Mercedes are a step ahead - how much is not clear, but some believe it is as much as a second a lap - and that the next bunch of Red Bull, Ferrari and Williams are closely matched.
"Will the next four days consolidate or change those impressions?"
How they stand
Well, it's fair to say no one's mind has been blow by the action on the track thus far. But this is F1 and there's always a good tale around the corner...
For now, here's a look at the times as things stand two and a quarter hours into this ninth day of pre-season testing.
1. Lewis Hamilton (Mercedes) 1:24.881 (29 laps)
2. Daniil Kvyat (Red Bull) 1:25.987 (26)
3. Felipe Massa (Williams) 1:26.399 (19)
4. Romain Grosjean (Lotus) 1:27.525 (24)
5. Marcus Ericsson (Sauber) 1:27.802 (27)
6. Kimi Raikkonen (Ferrari) 1:29.590 (18)
7. Jenson Button (McLaren) 1:31.479 (7)
8. Carlos Sainz (Toro Rosso) no time (24)
De Ferran warning for McLaren and Alonso
Here's an interesting tale from our website last night that you may have missed.
The recuperating Fernando Alonso has been warned not to rush back to racing following the crash that left him concussed and forced him to miss this test.
Gil De Ferran, a winner of the Indianapolis 500, two-time IndyCar champion and a man who has suffered more than his fair share of concussions, told BBC Sport that returning too soon is "the worst thing a driver can do".
You can read Andrew Benson's full story here.
Pace-setter
Mercedes tweet: Two hours into the first day of the final test! 26 laps on the board for @LewisHamilton, 1:24.881 his best so far!
F1 journalist Dan Knutson in Barcelona
"The paddock is pretty deserted this morning - almost no sponsor guests are around."
Benson's take on McLaren's troubles
Andrew Benson
Chief F1 writer
"While the thoughts and best wishes of everyone are with Fernando Alonso as he recuperates at his home in Oviedo, 560 miles away, following his crash last Sunday, his McLaren team will inevitably remain a major focus of attention at the Circuit De Barcelona-Catalunya these next four days.
"Quite apart from missing their star driver, McLaren are so far in no sort of shape for the new season, after eight severely troubled days of testing.
"Last week's test was ruined by continuing faults with a relatively minor part - a seal on the motor generator unit recovering energy from the rear axle. A fourth version of this part is to be tried this week in an attempt to give the team some proper running at last."
Button on the board
Jenson Button has finally posted a lap time in the McLaren Honda, but his 1:31.479 - some 6.5secs down on Hamilton - suggests the team is still finding its legs on this first morning.
McLaren chairman Ron Dennis has been pictured in the back of the team garage. There's a man with plenty to think about.
New Force India en route to Barcelona
Force India have endured a troubled pre-season programme, skipping the first test in Jerez and then running a modified 2014 car in the second.
However, the team said this week that they would have their new challenger on track this Friday - and a tweet this morning has confirmed the new motor is boxed up and on its way from Silverstone to Spain.
"Our VJM08 is currently on its way to Barcelona; we'll be aiming for action on Friday afternoon and on the two full final days," the team said.
The team added that after the car is "completed on Friday morning", Nico Hulkenberg will be first behind the wheel. The German and Sergio Perez will share driving duties across the three days.
More on new-look Toro Rosso
Andrew Benson
Chief F1 writer
"All teams will have upgrades at this test, with a view to preparations for the first race. And Toro Rosso's is the first to break cover.
"The team have a new front-nose design, replacing the smooth, rounded nose design of before with a Williams/Red Bull-Style stump. The aim is to improve airflow under the car - and thereby increase downforce - while satisfying the new safety regulations."
"I'm where I love being"
As you're reading this blog there's every chance you dream - or once dreamt - of being an F1 driver.
So it's good to know Lotus' Romain Grosjean is fully appreciative of where he is today.
"Wishing you all a very good day!" he wrote on Twitter. "I'm where I love being, behind the wheel..."
Hamilton into the 1:24s
Meanwhile, that ominous-looking Mercedes is still pounding away. Hamilton improves to 1:24.881 on the medium tyre. That's around five-tenths off the pace team-mate Nico Rosberg managed on the same tyre on the final day of the second test at the same circuit.
Picture perfect Sauber
The Sauber team have posted this rather lovely picture on Twitter of Marcus Ericsson sitting patiently at the end of the Barcelona pit lane waiting to light up his tyres with a practice start.
As you can see, it's a perfect day for a good hoon around the Circuit de Catalunya.
Low-key start for Button and Honda
After a trouble pre-season testing programme so far and driver Fernando Alonso sidelined, there's plenty of interest in the progress of the new McLaren Honda car.
Jenson Button, complete with new Niki Lauda-esque helmet, is the man behind the wheel today and it's been a low-key start so far - just two installation laps from the Briton and no time on the charts as yet.
Mercedes the benchmark - Horner
With Hamilton top of the pile now seems as good as time as any to bring you a line from Red Bull boss Christian Horner, who believes Mercedes are still the team to beat in Formula 1, describing them as "the benchmark".
Horner was speaking on a special F1 edition of A Question of Sport broadcast last night.
If you missed last night's show, it will be broadcast again on Friday, at 19:30 GMT on BBC One.
Hamilton tops early timesheets
Lewis Hamilton has set the early pace inside the first hour, the Mercedes driver quickest in a time of 1:26.437 on the medium tyre, around four-tenths up on Williams' Felipe Massa, who is on the hard.
New package for Toro Rosso
Toro Rossotweet: "Carlos is back in the garage.. Spot the changes on our car. We're running a new package #GOTOROROSSO"
Early pace setter
Kimi Raikkonen's settled in to a few decent laps so far.
He opens up with a 1:32.451 and immediately wipes it off with a 1:30.7, a 1:30.692, a 1:30.271 and a 1:30.526.
Restart
Kvyat's RB11 is pushed to the garage after a nightmare start for the Russian. Green flags are out and Lewis Hamilton joins Raikkonen out on the track.
Red flag
Not a great start to the day for Kvyat. He sets off for an installation lap and only gets as far as the end of the pit lane before grinding to a halt.
Out comes the red flag.
Today's likely lads
So, as well as Kimi Raikkonen, who as Lee McKenzie said has been first out on the track this morning, we're expecting to see seven other drivers go through their paces today.
Fresh from his red carpet appearance at last night's Brit Awards, world champion Lewis Hamilton will be heading out in his Mercedes, Jenson Button will put the McLaren through its paces, Marcus Ericsson will test out the Sauber, Romain Grosjean his Lotus, Daniil Kvyat in the Red Bull, Felipe Massa for Williams and Carlos Sainz in the Toro Rosso are all set to head out today.
Lie of the land
Andrew Benson
Chief F1 writer
"There is much to dwell upon as the final pre-season test of 2015 gets under way.
"Two weeks before the F1 circus sets up camp in Melbourne's Albert Park, teams will be introducing first-race upgrades to their cars these four days in Barcelona, and trying to tick all the boxes in terms of preparation.
"There will even be a new car to look at in the shape of the 2015 Force India.
"But before the cars are shipped off to Australia one question hangs above all others - will Fernando Alonso, arguably the sport's greatest participant, even be there?"
Post update
Lee McKenzie
BBC F1 pit-lane reporter
On Twitter: It's cold in Barcelona at the final #f1 preseason test and it's Raikkonen who is the first car out on track
They're off.....again
After the briefest of breaks, the teams and drivers are back in Barcelona.
It's the final four days of pre-season testing before the big off in Melbourne on 15 March.
With preparations for the season opener at Albert Park ramping up by the day, and crucial mileage still to be accumulated, we'll bring you all the latest twists and turns from the track.