London - Australia's Steve Smith has been ruled out of the last day of the second Ashes Test with concussion against England at Lord's, Cricket Australia announced on Sunday.
Smith was felled when a Jofra Archer bouncer struck his neck on Saturday.
"Steve has been closely monitored by medical staff overnight and this morning reported that after sleeping well, he woke with 'a bit of a headache and a feeling of grogginess'," said an Australia team spokesman.
The series is the first being played under the International Cricket Council's new concussion substitute regulations, part of the inaugural World Test Championship, which allow players who have suffered head or neck injuries to be replaced fully by a substitute, who was previously restricted to fielding alone.
Marnus Labuschagne, on the field as 12th man when play resumed Sunday, became the first concussion substitute in Test history after Australia's request was approved by match referee Ranjan Madugalle.
Labuschagne can now bat or bowl in Smith's place if required.
There are doubts whether star batsman Smith will feature in the third Test at Headingley starting on Thursday, with the spokesman saying "the short turnaround is not in his favour".
Smith was hit by an Archer bouncer when on 80 on Saturday and retired hurt.
He resumed his innings at the fall of the next wicket after 46 minutes off the field, and was eventually out for 92 -- the first time this series he had been dismissed for fewer than 100 runs.
The spokesman added: "Steve reported that his left arm which was also struck (by Archer) during his innings yesterday was 'much better'" and that as part of further testing on Sunday, Smith "demonstrated some deterioration".
"On that basis Steve has been withdrawn from the match by team doctor Richard Saw," the spokesman said.
Explaining how Smith had been allowed to resume his innings, he said: "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.
"Steve's fitness will be assessed on an ongoing basis.
"Steve will undergo a precautionary scan on his neck on Sunday."
Significantly, Smith was wearing a helmet without the additional stem guard neck protection introduced following the death of Phillip Hughes after his former Australia team-mate was hit by a bouncer in a 2014 domestic Sheffield Shield match in Sydney.
"He (Smith) just doesn't feel right (wearing a stem guard)," Australia coach Justin Langer said after stumps on Saturday.
"I know they came in after the tragedy of Hughesy. He might rethink it now after seeing what happened today, but you'd have to ask him that.
"At the moment, the players have a choice and I wouldn't be surprised if they become mandatory in the future."
Meanwhile Archer, criticised on social media for not showing enough concern for the stricken Smith, said Sunday he had been deeply worried when the batsman collapsed to the turf.
"You are trying to get a wicket first," Archer told BBC Radio.
"To see him go down, everyone stopped and everyone's heart skipped a beat."
Australia, bidding for their first Ashes campaign triumph on English soil in 18 years, lead the five-match series 1-0 after a 251-run win at Edgbaston last week.
In that match, former captain Smith marked his first Test since completing a 12-month ban for his role in a ball-tampering scandal in South Africa with superb innings of 144 and 142.
After rain delayed Sunday's start by over an hour, England resumed on 96-4 in their second innings, a lead of 104 runs.