Brown a doubtful starter for England
England fullback Mike Brown is unlikely to return to action in time for the crucial Six Nations clash against Ireland in Dublin on Sunday.
The 29-year-old suffered a setback in his recovery from concussion.
He felt a recurrence of his symptoms last 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.
"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 fullback, who has impressed in the opening two rounds of the Championship.
"I've got to be 100 percent 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 percent 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.
Rob Webber (Bath) will also join the squad to cover Tom Youngs, whose training on Tuesday will be limited following Leicester Tigers' Premiership game against London Irish on Sunday.
"Chris Pennell did well on the New Zealand tour and with Mike still in the early stages of the graduated return to play protocol we wanted another fullback to train along with Alex Goode," Lancaster said.
"Courtney Lawes, Geoff Parling and Tom Wood are all expected to train tomorrow [Tuesday] and we'll have some selection decisions to make over the next 24 hours before we release some players back to their Premiership clubs on Tuesday."
In addition, Manusamoa Tuilagi (Leicester Tigers) will be in camp continuing his recovery from injury.
England squad:
Forwards: Dave Attwood, Kieran Brookes, Calum Clark, Dan Cole, Alex Corbisiero, Tom Croft, Nick Easter, Dylan Hartley, James Haskell, George Kruis, Courtney Lawes, Joe Marler, Geoff Parling, Chris Robshaw, Mako Vunipola, Billy Vunipola, Rob Webber, Tom Wood, Tom Youngs.
Backs: Brad Barritt, Mike Brown, Luther Burrell, Danny Care, Danny Cipriani, Kyle Eastmond, George Ford, Alex Goode, Jonathan Joseph, Jonny May, Jack Nowell, Chris Pennell, Manusamoa Tuilagi, Billy Twelvetrees, Anthony Watson, Richard Wigglesworth, Ben Youngs.
Date: Sunday, March 1
Venue: Aviva Stadium, Dublin
Kick-off: 15.00 (15.00 GMT)
Referee: Craig Joubert (South Africa)
Assistant referees: Jérôme Garcès (France), Mathieu Raynal (France)
TMO: TBC