England must enter Sunday's RBS 6 Nations title showdown against Ireland without their most ferocious competitor after Mike Brown was ruled out of the Dublin encounter with concussion.
Brown was knocked unconscious during an accidental collision in the 47-17 round two victory over Italy and has run out of time to successfully complete the return to play protocols.
The 2014 player of the Six Nations, who had already suffered one setback in his recovery last week, woke up on Wednesday morning with a "mild headache" having completed a running session the previous morning.
Alex Goode will start at full-back at the Aviva Stadium and Brown - an instigator of the recent tunnel stand-off with Wales at the Millennium Stadium - must now switch his attention to proving his fitness to face Scotland on March 14.
"Mike woke up not feeling 100 per cent," backs coach Andy Farrell said.
"The right and proper thing to do was to make the call on him. His health is the main priority here and we need to get him right for the next game.
"The symptoms aren't too severe whatsoever. He's fine in himself and is chirpy enough, but it just isn't worth the risk because his health comes first."
Brown's absence has robbed England of their second most experienced player in a backline that is light on caps.
The fiery 29-year-old was outstanding in the victories over Wales and Italy and the loss of his aggression is a blow to the title favourites' hopes of dispatching Ireland and taking a significant step towards claiming a first Six Nations crown under Stuart Lancaster.
Goode won the last of his 16 caps against Wales 12 months ago, a victim of Brown's emergence as one of England's most steely and reliable performers, but Farrell views the Saracen as a highly-capable deputy.
While lacking Brown's tenacity and physical edge, Goode offers a more rounded playmaking option.
"Mike has been in great form. He's a guy who stands up in pressure situations and performs extremely well. With the young backline we have, he's been a massive help for us,'' Farrell said.
"But Alex is an experienced guy himself who has been waiting for his chance for some time now. His form for Saracens in the big games against Munster and Clermont and in recent matches has been exceptional.
"Alex is super excited. All you ask for a player who has been waiting in the wings is for them to be ready and we feel he's certainly ready for us.''
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