England have called up Nick Easter to their World Cup squad after Billy Vunipola was ruled out of the tournament by a knee injury.
The No 8 sustained a knee ligament strain in England's 28-25 defeat to Wales on Saturday and is expected to be out for up to 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.
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.
Meanwhile, England flanker Tom Wood has been cleared to play against Australia on Saturday after being warned, and not cited, for striking Wales full-back Liam Williams.
Williams' evening was ended in the 67th minute when he was struck on the head by Wood's shin as the Northampton forward attempted to kick a loose ball towards the Welsh line.
The match citing commissioner, Maurizio Vancini, has issued a warning that equates to a yellow card in sanction, clearing Wood to face the Wallabies at Twickenham.
Williams received several minutes of treatment before being driven from the pitch on the medical cart and is a doubt for Thursday's clash with Fiji, although the initial signs are that he is making encouraging progress from the concussion he sustained.
England will be relieved to have their first-choice blindside flanker available to face the Wallabies, knowing that only victory will be enough to save their World Cup following the loss to Warren Gatland's side.
"Under the Rugby World Cup 2015 disciplinary programme, a citing commissioner warning equates to a yellow card in sanction," read a statement released by tournament organisers.
"A combination of three yellow cards or citing commissioner warnings accumulated during the tournament will result in a hearing. Wood is therefore free to play in England's next Pool A match against Australia."
Wood, who has no disciplinary record to speak of, expressed his remorse over the incident while insisting the action was reckless and not deliberate. It is a view that appears to have been shared by the citing commissioner.
"The outcome was not something I intended obviously. It's something I'm not proud of, but there was no intent there," Wood said.
"Would I have done anything differently? Probably not because the ball was bouncing at my feet and I was at full tilt looking for the ball. It was unfortunate I caught him sweet with the shin.
"It's hard to watch when someone is in that condition on the floor. We all take our turn at that. I'm sorry for the outcome 100%. I'm told he's on his feet and moving around as normal which is quite a relief.
"He was in the shower when I went into Wales' changing room, but I caught up with a few of their players and they assured me he was on his feet and feeling better and moving around OK.
"That's a huge relief because there's an element of guilt there. It wasn't intentional and I hate to see a player leave the field that way."
