Sculthorpe has been dogged by injury all season

Sculthorpe has been dogged with injury since the start of the campaign, missing several weeks of action with groin and hamstring problems.

But this latest setback centres on another long-term knee injury.

Great Britain vice-captain Jamie Peacock is expected to take over as skipper for the Tri Nations.

Sculthorpe is the third withdrawal from coach Brian Noble's initial 40-strong Great Britain squad.

Wigan full-back Kris Radlinski, who was absent for last year's Tri-Nations Series, has been ruled out for the rest of the season in order to have knee surgery.

I'm pleased that he's been ruled out now so that we can be spared playing with an unfit captain From PT Have your say on Five Live

And Hull prop Paul King has told Noble he needs to have a wrist operation at the end of his club's Super League campaign.

But Sculthorpe's withdrawal is the biggest blow by far.

Bradford forward Peacock said: "Scully is a world-class player who performs as well in a Great Britain jersey as he does for Saints.

"Four or five years ago we might have struggled but we have more strength in depth these days and that softens the blow."

St Helens coach Daniel Anderson was also philosophical.

"It's disappointing to lose a player of his calibre at a crucial part of the season," said Anderson.

At Saints we do have a strong squad and I have complete confidence that the players will take on the added responsibility Daniel Anderson St Helens coach

"At Saints we do have a strong squad and I have complete confidence that the players will take on the added responsibility.

"Paul is an integral part of the club's long-term plans and we want him to be in the best possible condition to ensure that he can continue to play for St Helens and Great Britain for years to come."

Sculthorpe has twice won the prestigious Super League Man of Steel award and scored his 1,000th point for St Helens earlier this season.