Easter 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,” England 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 No 8, who has 52 Test caps and was involved in the 2007 and 2011 World Cups, was due to join the squad at their Surrey training camp yesterday 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.”
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. “Nick is an experienced guy who has been there and done it” he said. “He’s a smart guy who understands what pressure rugby is. He goes straight into contention.”
l England flanker Tom Wood has been cleared to play against Australia 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.
The match citing commissioner, Maurizio Vancini, has issued a warning that equates to a yellow card in sanction, clearing Wood to face the Wallabies.
“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.”