Wales captain Matthew Rees has been ruled out of the Rugby World Cup as the hooker will undergo surgery on his "severely painful" neck injury.
The British and Irish Lions front-rower, 30, has had intensive treatment and pain-killing injections for what is believed to be a disc-related problem.
Ospreys hooker Richard Hibbard has been added to the initial World Cup squad.
"The safety and longevity of Matthew's career is paramount," said Prav Mathema, the Welsh Rugby Union's medical performance manager.
"We are all very disappointed that Matthew will not be with us in the World Cup but his health is at the fore of our decision.
"Following a series of injections and intensive treatment and rehabilitation, the decision has been made to omit him from the squad to allow him to undergo surgery.
"The medical teams of both the Scarlets and the WRU concluded unanimously to proceed with surgery."
Gatland had hoped Rees' surgery could be delayed until after the tournament.
"There are more important things in life than rugby," said Wales assistant coach Shaun Edwards.
"And Matthew is in such pain it has to be second to his life.
"We are gutted he will not lead us into the World Cup but he has to take the doctor's advice because you can't live like that.
"He can't sleep and is in constant agony."
Rees, however, is expected to be fit for the 2012 Six Nations.
Hibbard now comes into World Cup contention as he joins Huw Bennett, Lloyd Burns and Ken Owens in the battle for one of the hooker berths in Gatland's 30-man squad when the Wales coach announces it on 22 August.
Sam Warburton has skippered Wales in Rees' recent absence and the 22-year-old flanker is front runner to captain Wales at the World Cup.
Warburton is due to be rested on Saturday in the final warm-up, against Argentina.
Wales' preliminary World Cup squad
Forwards: Scott Andrews, Gethin Jenkins (Blues), Lloyd Burns (Dragons), Huw Bennett, Ryan Bevington, Richard Hibbard, Adam Jones, Paul James (Ospreys), Ken Owens (Scarlets), Craig Mitchell (Exeter), Bradley Davies, Sam Warburton, Martyn Williams (Blues), Luke Charteris, Danny Lydiate, Toby Faletau (Dragons), Ryan Jones, Alun Wyn Jones, Jonathan Thomas, Justin Tipuric (Ospreys), Josh Turnbull, (Scarlets),Gareth Delve (Melbourne Rebels), Andy Powell (Sale Sharks).
Backs: Michael Phillips (Bayonne), Lloyd Williams (Blues), Tavis Knoyle (Scarlets), Jamie Roberts (Blues), James Hook (Perpignan), Jonathan Davies, Stephen Jones, Rhys Priestland, Scott Williams (Scarlets) Gavin Henson (unattached), Leigh Halfpenny (Blues), Lee Byrne (Clermont Auvergne), Aled Brew (Dragons), Shane Williams (Ospreys), George North (Scarlets).