Vunipola was taken off at half-time in the 39-10 win over Argentina on Saturday with ankle pain.

The 26-year-old is now a serious doubt for England's final group game against France this weekend.

"He's been in and out of the boot," said England's attack coach Scott Wisemantel.

"We rested him today [Tuesday] at training - at this stage he's available for selection, but if there's any doubt then we'll rest him.

"We'll make a call on Wednesday night on whether he's fit to play or not."

The younger Vunipola brother - elder brother Mako has just returned from injury himself - is arguably England's most important player, the only specialist number eight in Eddie Jones' 31-man squad and a man whose ball-carrying is essential to Jones' preferred way of playing.

Tom Curry switched from blind-side flanker to the back of the scrum during the second half against Argentina and could play there again if required, while Mark Wilson - who impressed there during last year's autumn internationals - is short of game-time having been injured for the first fortnight of England's time in Japan.

Open-side flanker Sam Underhill told BBC 5 Live: "Billy would be a massive loss but we'd be in a pretty good place with the strength in depth that we've got.

"Billy's a fantastic player and brings such a unique skill set to the game, but we've got a diverse group.

"The back row is about balance, and within that group any of the combinations would work pretty well."

Wisemantel confirmed that the results of a scan on Vunipola's injury are with the team's doctor, but a day after scrum coach Neil Hatley had told journalists Vunipola had not been in a boot, there has been no public word on what the scan may have shown.

Captain Owen Farrell and hooker Luke Cowan-Dickie also missed training, but are expected to recover in time to feature in Yokohama.

Wisemantel said: "Owen's just had a little bit of a stomach virus, both he and Luke.

"I don't know where they've gone, they've probably gone to some little noodle place and got unlucky."

England are already through to the quarter-finals and will top Group C if they prevail against France.

That is likely to give them a quarter-final against Australia in Oita in 11 days' time, four years on from being knocked out of the World Cup at Twickenham by the Wallabies.

With New Zealand possible semi-final opponents for the winner of that tie, some have suggested that defeat by France - and a route into the opposite side of the draw, where Wales and potentially South Africa lie in wait - may be a preferred option.

But Wisemantel said: "Hand over heart, the next big thing is this weekend. It's such a tough tournament if you look far ahead you'll get lost.

"I don't think there's right or wrong side of the draw - you've got to adapt."