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England scrum-half Jack van Poortvliet will miss the Rugby World Cup because of an ankle injury which requires surgery.

Northampton Saints scrum-half Alex Mitchell has been called up as a replacement.

Van Poortvliet, 22, sustained the injury during England's 19-17 World Cup warm-up win over Wales.

England head coach Steve Borthwick wished the Leicester Tiger "a swift and successful recovery".

"It's really bad luck for Jack, and we send him our very best wishes," he added.

Van Poortvliet, who was named in England's 33-man World Cup squad last week, was Borthwick's main starting scrum-half during the Six Nations and was in the XV for Saturday's win against Wales.

Fly-half Owen Farrell could also miss the start of the World Cup after his red card against Wales, meaning England may be forced to pick a new half-back pairing less than a month out from the tournament.

Mitchell, 26, was initially cut from the squad in June but now joins Ben Youngs and Danny Care as one of three scrum-half options.

England's next warm-up fixture is against Ireland in Dublin this weekend, with their first World Cup game against Argentina on 9 September in Marseille.

Analysis - Chris Jones, ´óÏó´«Ã½ rugby union correspondent

Since making his debut last summer, Jack van Poortvliet has established himself as England's number one scrum-half, so for him to be denied a chance to play at a first World Cup is a cruel blow.

Alex Mitchell comes into the squad and will provide zip and spark, although it is understood his game management - or lack of it - was a reason behind his original omission, so how he fits into Steve Borthwick's gameplan will be intriguing.

It will also be interesting to see who Borthwick starts at nine in England's two remaining August warm-up matches, with the experienced pair of Ben Youngs and Danny Care jousting for the shirt - a familiar narrative of the past decade.