London Irish scrum-half Tomás O'Leary is unlikely to play again this season because of a serious back problem.
The Aviva Premiership club announced 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 on 28 October.
"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 from Munster.
He 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.
This latest injury will also rule O'Leary out of inclusion in the Ireland squad for the upcoming Six Nations.