London Irish scrum-half Tomas O'Leary is unlikely to play again this season because of a serious back problem.
The Aviva Premiership club announced on Tuesday that 29-year-old O'Leary has undergone surgery to his lower back for what they describe as a "degenerative pre-existing injury" that has deteriorated this term.
In a statement, Irish said: "Tomas has been unavailable for selection since his last appearance for London Irish against Harlequins at Madejski Stadium on October 28. He now faces a four to six months lay-off."
O'Leary, who has won 24 caps for Ireland, joined London Irish on a three-year contract at the start of this season.
He previously played for Munster and was selected to tour South Africa with the 2009 British and Irish Lions.
But he suffered a broken ankle while playing for Munster against the Scarlets just a month before the three-Test trip, and was then ruled out of the tour.
The Irish later added in a statement: "Tomas originally suffered an injury to his lower back in the Aviva Premiership match with Northampton Saints on October 6th and aggravated it during the first half of the Aviva Premiership match with Harlequins on 28th October.
"Following further medical examination the decision was taken that surgery was necessary to solve this problem.
"Tomas underwent surgery on 4th December. Given the serious nature of the injury and subsequent surgery, Tomas is not expected to be available for selection until April 2013 at the earliest."