England scrum-half Harry Ellis has been ruled out of the World Cup.
Ellis was injured against Bristol earlier this month
The Leicester scrum-half ruptured his left anterior cruciate ligament against Bristol on 5 May and will have surgery on his knee later this week.
England head coach Brian Ashton said: "This is a major blow to Harry, to Leicester given that they are aiming for a Treble, and to England.
"He was a key figure in the Six Nations and will be sorely missed. I wish him all the best for a full recovery."
Ellis had to sit out Leicester's Premiership final win over Gloucester on Saturday and will also be sidelined when the Tigers meet Wasps on Sunday in the Heineken Cup final.
Leicester boss Pat Howard said: "Harry has had an excellent season and an impressive Six Nations but now he will miss the Rugby World Cup and two big finals with Leicester.
"I really feel for him and we all look forward to having him back."
Ellis was rated by many as England's most consistent player in the 2007 Six Nations and appeared to have made the number nine shirt his own, but he is now likely to be out for six to eight months.
The leading contenders to replace him for England are Bristol's Shaun Perry, Gloucester's Peter Richards and Andy Gomarsall of Harlequins.
Richards said the injury was bad luck on Ellis, who had been playing well, but it created an chance for someone else to replace him.
"It's tremendously disappointing for him to be injured but the fact that Harry won't be there means there is that starting place up for grabs," he said.
"There are the likes of Andy Gomarsall and Shaun Perry who, like myself, wish to have that position. All three are in South Africa and I'm sure we'll be involved in the World Cup warm-up squad.
"England have unfortunately been deprived of one of their better players - but Harry's loss is someone else's opportunity."
Leicester were forced to turn to third-choice scrum-half, Frank Murphy, against Gloucester on Saturday, but the Tigers may have Scott Bemand back from injury in time to face Wasps.