Steve Smith out of third Test in massive Ashes blow for Aussies

Steve Smith has been ruled out of the third Ashes Test.

Australia have confirmed Smith, who suffered a delayed concussion after being struck on the neck by a Jofra Archer bouncer at Lord's, will not take the field when the series continues on Thursday.

Smith failed to warm up on Tuesday, when the touring party had their first training session since arriving in Leeds.

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Smith was in far better spirits on Tuesday, compared to Sunday, when he became the first concussed cricketer to be substituted out of an international match.

Smith was desperate to play after playing cards on the team bus to Leeds and feeling up to walking around the city on Monday.

The former skipper had a long chat with coach Justin Langer while inspecting the pitch and talked with team doctor Richard Saw, cutting a dejected figure in both conversations.

The absence of the talismanic run glutton is a major blow to Australia's hopes of winning in Leeds. Victory would ensure they retain the urn.

Marnus Labuschagne, having delivered a match-saving 59 while filling in for Smith on day five at Lord's, will surely need to step up again.

The pressure will also rise on Travis Head, who has been Australia's best-performing batsman on the tour outside Smith.

"It (the Lord's draw) showed that without Steve, we were able to get the job done which is a positive," Head said.

Aussies confident of coping without Smith

"But obviously, Marnus came in and played beautifully. He has been fantastic over here in the county season - got a lot of hundreds.

"Wadey got a hundred at Edgbaston, so I think we have got guys who are in form ... we have got the batting in the sheds that can do the job."

Cricket Australia's concussion and head trauma policy dictates the only opinion that matters in Smith's selection battle is that of Dr Saw, which the right-hander and captain Tim Paine have both publicly conceded.

"The doc has been doing a fantastic job so far on tour with everyone. He had a busy week," Head said.

"I know he's looking after him pretty well - he is in good hands."

Smith's setback increases the likelihood of under-pressure Cameron Bancroft retaining his spot at the top of the order.

However, selectors are likely to consider promoting Mitch Marsh or Marcus Harris in place of Bancroft.