England have called up Nick Easter to their World Cup squad after Billy Vunipola was ruled out of the tournament by a knee injury.
Vunipola suffered a ligament strain in Saturday's 28-25 defeat by Wales at Twickenham and a scan has revealed he will be sidelined for six weeks.
"It's so unfortunate for Billy but the scan shows an injury which means he can't take any further part in the World Cup," head coach Stuart Lancaster said.
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Easter could be promoted straight into the back row against Australia on Saturday if Ben Morgan is ruled out for another match by his knee problem.
The 37-year-old number eight, who has 52 Test caps and was involved in the 2007 and 2011 World Cups, will join the squad at their Surrey training camp on Monday evening.
"Billy worked really hard and committed everything to the last three months," Lancaster said.
"He is a young man with a bright future ahead of him and I know he will be an integral part of this team for years to come.
"Nick was unlucky to miss out on final selection. He is a specialist number eight and an experienced player. We know he will make a big contribution."
Vunipola was one of England's best performers against Wales, casting aside his error-strewn displays in the warm-up matches to rediscover the form he showed in the World Cup.
Easter just missed out on selection for the original 31-man squad and was superb against France in the second warm-up match.
Backs coach Andy Farrell admits England are "devastated" to have lost Vunipola, but acclaimed Easter as a quality replacement.
"We're all devastated for Billy. He has a four to six week injury on his knee. We all know what kind of player Billy is. He was excellent against Wales but unfortunately he got caught under a maul," Farrell said.
"Nick is an experienced guy who has been there and done it. He's a smart guy who understands what pressure rugby is. He goes straight into contention for Saturday."
Farrell revealed that Morgan is available for selection against Australia after resuming full training on Monday, Jonathan Joseph is progressing well and could be available, but Courtney Lawes and Ben Youngs must prove their fitness over the next two days.