Byrne is the sixth Welsh Grand Slam star to pull out of the tour

Wales coach Warren Gatland is suffering a summer tour injury crisis after three more Grand Slam stars withdrew from the trip to world champions South Africa.

Martyn Williams, Lee Byrne and Dwayne Peel have confirmed their absence and join centre Gavin Henson and scrum-half Mike Phillips on Wales' sick list.

Flanker Williams sprained his knee in the Blues win over Connacht while Byrne needs keyhole surgery on a knee injury.

Scrum-half Peel needs shoulder surgery while back-row Alix Popham is also out.

Veteran Lions star Williams, one of Wales' star men in their 2008 Grand Slam triumph, twisted his knee in a collision during Cardiff Blues' Magners League win at Connacht on Friday.

Wales physio Mark Davies confirmed: "Martyn sprained his knee on Friday during the Blues match with Connacht and was scanned on Saturday.

"The scan revealed torn ligaments and he is currently in a brace and awaiting further surgical opinion early next week but the injury rules him out of the summer tour.

"Dwayne has a shoulder problem which is unstable and in need of repair, once he undergoes his operation he will be five months-plus in recovery.

"Gavin has had a successful operation on his ankle and he will begin his rehabilitation in approximately six weeks time."

Byrne played in the Ospreys' Liberty Stadium defeat to Edinburgh and will have surgery now the Ospreys season is over.

Byrne's absence is a blow for Gatland as the 27-year-old was one of Wales' top performers in their second Grand Slam triumph in four years as he scored three tries in their all-conquering campaign.

But the Ospreys received the full backing of Wales' medical team before deciding that Byrne will have an operation, a procedure that will take place this week.

In the best interests of Lee, the decision has been taken to use the summer break to deal with the problem Ospreys physio Chris Towers

Scarlets rookie Morgan Stoddard is set to step in for Byrne at full-back but the Ospreys expect Byrne, who played in their Liberty Stadium defeat to Edinburgh on Friday, to be ready for the start of next season.

Ospreys physio Chris Towers explained: "Lee has been suffering intermittent pain and swelling of his left knee due to a cartilage defect of his patella femoral joint.

"Although this hasn't been limiting his ability to perform on the rugby field, it had started to impact on certain aspects of his training.

"So in the best interests of Lee, the decision has been taken to use the summer break to deal with the problem by way of arthroscopic surgery to resolve the issue.

"By taking this action at this moment in time we are ensuring that disruption caused by the injury are kept to a minimum and we are allowing him extra recovery time post op.

"That will ensure that he is able to train and prepare properly for the 2008-09 season and the challenges that lie ahead."

Byrne's imminent operation means he is the fourth Grand Slam star to withdraw from Wales' two-Test summer tour in June.

Phillips is out until October with a damaged cruciate knee ligament, Henson has undergone surgery on an ankle problem and second row Popham is house-hunting in France as he completes his move to Brive from the Scarlets.

Popham admitted: "I'm taking the family over to France for a week in about two weeks time to look for houses, find a school for my daughter Holly and generally get a feel for the area.

"It is disappointing that I've had to put Wales on the back-burner for a while, it was the hardest decision I've had to make.

"But if I had have gone on tour, it would have meant sorting my move to France in two weeks which would have been ridiculous.

"I spoke to Warren and he understood and wished me good luck."

The back-rower, 28, will not be replaced in Wales' training squad.

Wales centre Sonny Parker, however, has been cleared to join Warren Gatland's Whites of Wexford pre-South Africa tour training camp this week after recovering from a knee injury.

Wales will announce their full South Africa tour squad on 14 May and will take on the Springboks in two tests in Bloemfontein on 7 June and Pretoria a week later.

Wales training squad:

Backs: Morgan Stoddart, Jamie Roberts, Shane Williams, Tom James, Sonny Parker, Tom Shanklin, Stephen Jones, James Hook, Andy Williams, Warren Fury.

Forwards: Duncan Jones, Gethin Jenkins, Rhys Thomas, Adam Jones, Matthew Rees, Huw Bennett, Ian Gough, Alun Wyn Jones, Ian Evans, Jonathan Thomas, Ryan Jones.