England prop Dan Cole has been ruled out of the rest of the Six Nations because of a neck problem.
Cole has a bulging disc in his neck which is trapping a nerve.
The Leicester tighthead will now need a period of rest, though it has not been confirmed how long he will be out of action.
The problem was discovered then he experienced weakness during a gym session at St George's Park last week.
England head coach Stuart Lancaster said: "It's massively disappointing for Dan, Leicester and England but the important thing is that this has been diagnosed and that Dan gets the appropriate rest and treatment."
Cole has only missed two internationals since Lancaster took over as coach, and those were because he was away with the British & Irish Lions.
His injury is a big blow for England - he has been the cornerstone of an improving pack in the last 12 months.
Sale Sharks' Henry Thomas and Bath's David Wilson are the likely contenders to replace him, though the latter has only just returned from injury.
Thomas could be the man to start at Twickenham against Ireland in Saturday's crunch Six Nations match.
Lancaster added: "Our medics are working closely with Leicester's to come up with a programme that will ensure Dan returns in the best possible shape.
"In the meantime, it's great to have David Wilson back, as well as Henry Thomas - who has trained well and came off the bench against Scotland. This will be an opportunity to develop experience in this area."
Leicester director of rugby Richard Cockerill said: "It's very disappointing for us and for England - and obviously for Dan - but his long-term fitness is more important.
"He'll have time to get over this issue and come back when the medical teams are satisfied with his progress."
Who should replace the injured Dan Cole at tighthead prop for England's Six Nations game against Ireland?