Steve Smith was struck by a Jofra Archer bouncer in the first innings at Lords and was replaced by 'concussion sub' Marnus Labuschagne for the second innings.
Australia batsman Steve Smith has been ruled out of the third Test at Headingley due to the concussion he suffered at Lord's, Cricket Australia said on Tuesday.
Smith, who made two centuries in the first Test at Edgbaston, was struck by a Jofra Archer bouncer in the first innings at Lords and was replaced by "concussion sub" Marnus Labuschagne for the second innings.
It was the first time a player in international cricket was substituted under the new concussion rules.
Initially, Smith passed a concussion Test and was allowed to return to the field and complete his innings, but his condition worsened the following day.
Brain injury charity Headway said it was "incredibly dangerous" for Smith to resume his innings.
"You need to take an 'if in doubt' approach," Luke Griggs, Headway deputy chief executive, said.
"With concussion, the vision can be blurred and the brain can be slow at processing information. That leads to delayed reaction times and is just incredibly dangerous."
Smith scored 142, 144 and 92 in his three innings in the Ashes series and is ranked second in the ICC Test batting rankings.
Labuschagne is the most likely option to replace Smith for the Headingley Test starting on Thursday, having made 59 as Australia battled on Sunday to a draw which retained their 1-0 lead in the series.
Smith has two weeks to recover and prepare for the fourth Test in Manchester, which begins on September 4. Australia also play a three-day tour match in Derby from August 29-31.