Danny Aeberhard
Europe regional editor, BBC World Service

So far, the level of knock-on disruption to other European airports, caused by the closure of Heathrow, has been relatively limited.

Apart from cancellations of scheduled flights to and from Heathrow itself, some European airports have seen Heathrow-bound flights diverted. They include the following:

• Five such services landed at Charles de Gaulle Airport in Paris, one of the busiest hubs in Europe after Heathrow. Two had come from Singapore - a Qantas and a Singapore Airlines flight. The others were a Qantas flight from Perth in Australia, an Emirates one from Dubai and a RwandAir one from Kigali
• Seven flights have had to be diverted to Schiphol Airport in Amsterdam. They are Delta flights from Detroit, Boston, and Atlanta in the US, two Cathay Pacific flights from Hong Kong, a British Airways flight from Riyadh, and a Malaysia Airlines flight from Kuala Lumpur
• Frankfurt has seen six diverted planes land. These were a Singapore Airlines service from Singapore, an Air India flight from Delhi, a Gulf Air flight from Bahrain, an Etihad flight from Abu Dhabi and two services from Doha - one a Qatar Airways service, and the other a British Airways one

Meanwhile, Europe's most westerly international airport, Shannon in Ireland, has seen six diverted transatlantic flights land. They are flights from Atlanta, Boston, Orlando and Newark in the US, Toronto in Canada and one from the capital of Barbados, Bridgetown.

Image source, Getty Images

Prime Minister Keir Starmer has just posted an update on social media about the fire at the substation near Heathrow Airport.

"I know the situation in Heathrow is causing distress and disruption, especially for those travelling or without power in their homes," he says.

"I'm receiving regular updates and I'm in close contact with partners on the ground. Thanks to our emergency workers for keeping people safe."

Tom Finn
Visual Journalism Team

At 08:00 UTC on a normal Friday, there would usually be about 200 flights in the air headed for Heathrow.

Here’s a snapshot of air traffic that was bound for Heathrow a week ago at 08:00 UTC on Friday, 14 March.

And here’s traffic at the same time this morning as flights were grounded or diverted to other airports.

Why is Heathrow closed?

A fire at the North Hyde electrical substation in Hayes, west London, knocked out the power supply to Heathrow and other businesses and homes in the surrounding area.

The cause of the fire is not yet known, but emergency services were first called to the scene at 23:23 GMT on Thursday.

Energy Secretary Ed Miliband told the BBC the fire appeared to have knocked out a backup generator as well.

How long will the airport be closed?

Heathrow has said it doesn't "have clarity on when power may be reliably restored", and that it will be closed until 23:59 tonight.

“We will provide an update when more information on the resumption of operations is available," it says, also warning that travellers can expect "significant disruption over the coming days".

Are Gatwick flights cancelled?

No, London Gatwick says it is operating as normal - but some flights originally bound for Heathrow have been diverted to Gatwick.

Our experts will be answering more of your questions at 12:30 GMT live in this page, and you can also get in touch with us here.

Daniel Sandford
Home Affairs correspondent

The BBC understands that counter-terrorism officers from the Metropolitan Police will be involved in the investigation into a fire at a electricity substation near Heathrow.

This is not because of any direct evidence of sabotage, but because of their skills and the need to quickly rule out foul play.

Their involvement was described by a source as "belt and braces" and a "precautionary measure".

Jonathan Smith from the London Fire Brigade is asked about media reports that the Metropolitan Police's counter-terrorism unit is investigating the fire.

The Times reports, external that detectives are being deployed as a precautionary measure rather than an indication of foul play.

The BBC has contacted Met Police for comment.

Smith says the London Fire Brigade is unable to comment on the Met's investigations. However, he says that the Met Police are investigating the nature of the fire with support from the fire brigade.

Smith goes on to say that 29 people from neighbouring properties were evacuated over the fire, adding that around 150 people in total were evacuated to a rest centre.

He says the fire brigade will continue to have a presence in the area throughout the day.

Fire safety and fire investigation officers are working with the Met Police to establish the nature of the fire, he adds.

There remains no power at Terminal 2 or Terminal 4 at Heathrow Airport, Smith says.

We're now hearing from the London Fire Brigade, as deputy commissioner Jonathan Smith addresses reporters with an update on the fire at the electricity substation near Heathrow Airport.

The fire involved a transformer comprising 25,000 litres of cooling oil that was fully alight. 10% is still alight, he says.

Met Police declared it a major incident, he adds, saying that firefighters worked under challenging conditions.

Press watch live above to follow along. We'll bring you more updates shortly.

Several schools in Hayes are closed today due to the fire at the North Hyde Electricity Substation, according to a statement from Hillingdon Council.

The schools that will be closed throughout the day are:

• Nestles Avenue Early Years Centre
• Pinkwell Children's Centre
• Pinkwell Primary School
• Botwell House Catholic Primary School
• Dr Triplett's C of E Primary School
• The Global Academy

Your Voice, Your BBC News
Perisha Kudhail

Alex Money from Oxford was due to land in London today.

Instead his flight was rediverted from Cape Town to Barcelona, where he has just stepped off the plane after being on the tarmac for over two hours.

“It’s my birthday today. Not how I imagined spending it”, he says

Despite the change of plans, Alex says the full flight still seemed “remarkably good humoured despite the situation.”

He’s now waiting at the terminal in Barcelona for his bags and hoping for the best.

While passengers have been left frustrated by cancelled flights, we've also been hearing from residents who had to evacuate their homes in Hayes as a result of the fire.

Katrina Clements tells BBC Radio London how she and her husband were asleep when they heard knocking at the door and were told to leave their home.

Once she'd left, Katrina says she saw "a lot of people on the road" who had some from nearby to look at the fire.

"We just want to go home," she says.

Since evacuating, the Hayes resident adds that she's had little communication from authorities, though she says the council have tried to source hotel rooms for the night.

As we've been reporting, at least 1,351 flights to and from Heathrow Airport could be cancelled today, according to flight tracking website Flightradar24.

No flights are landing at Heathrow and planes that are already on the way there are being diverted to other airports, or back to their origin.

The video below compares today's air traffic in the vicinity of Heathrow to yesterday's.

As we've been reporting, the London Fire Brigade has said the fire at the North Hyde Electricity Substation in west London is now "under control".

They are expected to remain on the scene throughout the day to assist the National Grid in assessing the site.

Let's take a look at what we know, and don't know, so far:

• The London Fire Brigade received the first report about the fire at 23:23 GMT
• It sent 10 fire engines and around 70 firefighters to respond to a burning transformer within the substation
• Firefighters safely evacuated 29 people from neighbouring properties and a further 150 people were evacuated as a precaution
• More than 4,900 homes have been left without power
• There have been no reports of any injuries and the fire was under control by 06:28 GMT
• It's still unclear what caused the fire - Energy Secretary Ed Miliband described it earlier as an "unprecedented event"

Image source, UK Pool

Image source, PA Media

We've referred a lot this morning to a substation that caught fire in Hayes. Here's a bit more information about what that is.

Substations play an important part in the UK’s national electricity grid.

There are two types: transmission substations and distribution substations.

Transmission substations handle high-voltage electricity transmission over long distances, while distribution substations bring down the voltage for local distribution to homes and businesses.

Increasing and lowering the voltage of electricity for these purposes is one of the main roles of substations - and the process is done via a piece of equipment called a transformer.

According to the London Fire Brigade, a transformer within the electrical substation in Hayes caught fire.

Image source, Getty Images

Frustrated passengers might ask this morning why there was no backup plan at Heathrow in case the airport's usual power source failed.

Aviation expert John Strickland tells BBC 5Live that there are backup systems in place for key systems, but stresses the scale of the challenge of keeping an airport moving.

"There are just so many systems at an airport used by so many different players, not only the airport itself and the airlines, but handling agents, engineering companies, catering companies, you name it", he says.

"You could be talking about anything from switching on lights in a toilet to the far more serious stuff" he adds, citing "air traffic control functioning, aircraft manoeuvring around airports [and] baggage being loaded"

Despite this, Strickland suggests that the airport could "theoretically" do the minimum of landing an aircraft safely on its emergency systems.

And, he says the question about backup will be "the biggest topic of discussion once the dust settles".

More now from travel journalist Simon Calder, who has been speaking to BBC News.

He says the disruption at Heathrow "shows up once again how little slack there is in the system".

"This just points out the sheer lack of resilience. The one reason that there are people in Amsterdam, in Lyon in southern France, in Iceland right now is because there isn't enough room in south-east England to divert those planes," he says.

"The natural diversion airport would be London Gatwick," he says, adding that airport did take flights but then had to say "sorry, we're full, we are the busiest single-runway airport in the world".

Sean Dilley
Transport correspondent, reporting from Heathrow airport

We are as close to Heathrow Terminal 2 as we are allowed to be.

Some passengers have not taken advice that they should not turn up and not attempt to travel from Heathrow.

They’re being turned away on the bridge that leads to the closed airport

The skies are silent, the air traffic is nearly zero. About 10 minutes ago, two circling helicopters were seen and heard gathering pictures for the media, as the world watches the spectacle of an empty Heathrow Airport.

The last time I remember Heathrow being closed fully was in 2010.

Image source, Getty Images

The fire at a substation that has caused a massive blackout at Heathrow Airport has been brought under control - and aerial footage shows thick smoke billowing from it.

Fire crews can be seen working nearby, and planes can be seen parked at the airport.

Michael Race
Business reporter

With Heathrow being such a busy and important airport for passenger journeys and global trade, questions are being asked whether it has back-up systems in the event of power cuts.

The BBC understands that Heathrow does have back up power for its key systems, but kickstarting these alternative power supplies for the whole airport takes time.

A source says it isn’t possible to switch the power back on immediately.

And even once the power is back one, there are countless systems which need to be rebooted and checked to ensure they are working properly and are stable.