London - England fullback Mike Brown is unlikely to return to action in time for the crucial Six Nations clash against Ireland following a setback in his recovery from concussion.
The 29-year-old felt a recurrence of his symptoms on Friday after being knocked unconscious in the 47-17 victory over Italy and having completed a weekend of rest has restarted the return to play protocols.
The earliest the Harlequins star will be able to resume contact training is Friday, the day head coach Stuart Lancaster announces his starting line-up for Sunday's showdown between the Championship's only remaining unbeaten teams in Dublin.
It leaves Brown -- who had an outstanding tournament last year scoring four tries -- with a race against time to prove his fitness knowing any further flare-up in symptoms will force him to start the protocols again and rule him out of contention.
"It's very early for me to pencil Mike in for a game at the moment," Lancaster said at the squad's Pennyhill Park training base on Monday.
"We don't know how it is going to play out because he's done stage one of the process on Monday and there could be a reaction. That will have an implication, big time.
"A lot will depend on how the next 24 hours go but a decision will have to be made within the next two days I'd say because the team need a chance to prepare with all the pieces in place.
"If he ticks the boxes successfully on a daily basis then there'll also be a conversation to be had with him about how he's feeling mentally in terms of the lack of training he's been able to do."
Concussion is the most commonly reported injury for the last three years according to the Rugby Football Union's injury audit. It is now the most pressing issue facing rugby union.
The trauma suffered by George North in the Six Nations opener against England - the Wales wing played on despite having been knocked out - has placed the handling of concussion by team medics under close scrutiny.
Lancaster insists no risk will be taken with the rehabilitation of his first-choice full-back, who has impressed in the opening two rounds of the Championship.
"I've got to be 100 per cent certain that he's right with the type of knock it was and also to go in the size of game that this is," Lancaster said.
"On Mike's behalf I want to be 100 per cent certain he's right. I'm not being cautious, we're just following the protocols to the letter. His welfare is our primary concern and I'm simply guided by what the doctor tells me.
Alex Goode, who won the last of his 16 caps as a replacement against Wales 12 months ago, is on standby to start against Ireland, with Chris Pennell also in the squad as specialist cover in the position.