England prop Mako Vunipola faces a race against time to be fit for the World Cup after being told he needs hamstring surgery.

Vunipola has been ruled out for at least three months with the injury sustained in Saturday's Champions Cup final victory over Leinster in Newcastle.

"Vunipola's hamstring injury requires surgery and will take a minimum of three months to recover," a statement issued by his club Saracens read.

Saracens were more upbeat over his outlook on Wednesday only for a visit to a specialist to reveal the true extent of the damage.

England fly to Japan on September 8 and play their opening match against Tonga two weeks later.

Even if Vunipola's recovery takes the minimum three months, he will be forced to miss two of the four warm-up matches.