More from Cricket Australia: "Despite the unfortunate nature of what has happened, the positive is that the concussion protocol, including the availability of the concussion substitute, which has been recently brought in has served its purpose.

"A player is no longer under pressure to take the field when he or she displays symptoms of concussion and a side is not disadvantaged having lost a player to a blow to the head or neck.

"Cricket Australia is proud to have been leaders in this area, having introduced concussion substitutes in domestic cricket and worked with the ICC to introduce them in international cricket."

Adam: Smith being ruled out could be akin to McGrath stepping on the ball in 2005.

Ben Stokes is on 29 now from 64 balls. Seriously what would England have done without him this summer?

Jonathan Agnew
BBC cricket correspondent

Look how fast the outfield is! Lyon was looking on in dismay.

Runs for Stokes, Four of them!

This was nothing more than a gentle push but Josh Hazlewood escorts the ball to the boundary. It wasn't the sharpest of fielding efforts and Nathan Lyon is not too pleased.

Mike: Oh come on. Go into one day mode. Score 120-150 quick and get Jofra steaming in at them. Be brave, Joe.

Steve Smith's symptoms have been described as "headache, dizziness, feeling slowed down, feeling in a fog, doesn't feel right, drowsiness".

England's batsmen are playing themselves in. Buttler plays a good drive at the fielder but that's the only real attack shot in Cummins' over.

It's official - Marnus Labuschagne approved by the ICC as Steve Smith's replacement. He's the first like-for-like concussion replacement in Test cricket.

It'll be a big decision for Root. Can you risk going 2-0 down in the hope of getting it back to 1-1?

Omar in Kent: With Smith ruled out England could take their chances with a 150-200 lead. He's the only one they can't get out below 50.

Australia will be able to use the new concussion substitute in the rest of this match. The sub will be able to bat in the second innings and bowl in this England innings if they want.

Just a single from Lyon's latest over.

Cricket Australia statement in full: "Steve Smith has been closely monitored by medical staff overnight. He slept well but woke up with a bit of a headache and feeling of grogginess. He reported his left arm is much better.

"As part of the Cricket Australia concussion protocol, repeat testing was performed this morning and demonstrated some deterioration, which is consistent with the emergence of the symptoms he was reporting.

"On that basis, he has been withdrawn from the match. The Australian team will lodge an application for the concussion substitute with the match referee.

"In terms of his availability for the next Test, it will be considered over the coming days but the short turnaround is not in his favour. He will be assessed on an ongoing basis and will have a precautionary scan on his neck on Sunday.

"Cricket Australia statistics show that 30 per cent of concussions in Australian cricket are delayed. It is not uncommon for players to pass their tests and feel well on the day of an injury and then display symptoms 24 - 48 hours later."

The Supreme Meme Esq: It'll be sad if Smith is ruled out for the 3rd Test. I may not like him, but I don't want to see him injured and the series will be poorer for it.

Steve Smith's availability for the third Test, starting on Thursday will be considered over the coming days.

Cricket Australia say the short turnaround is "not in his favour".

Well that changes things...

And on the field Ben Stokes gets another boundary. This wasn't quite how he planned it but the crowd are cheering nonetheless.

It's official.

Steve Smith has been ruled out of the rest of the match due to delayed concussion.

Edged!

... wide of Usman Khawaja at gully.

Ben Stokes collects four.

I don't think Stokes is going to be able to do anything too attacking against Lyon. There is plenty of roughed up patches for the balding spinner to aim for. Stokes gets his first run with a slightly edgy push.

Geoffrey Boycott
Ex-England batsman on BBC Test Match Special

If anyone is going to win, I favour Australia. That's not to say England can't win - but they'll have to play exceptionally well.

Jonathan Agnew
BBC cricket correspondent

What's interesting about today is that there is something there for every bowler.