Billy Vunipola (centre) has undergone knee surgery ahead of the Rugby World Cup

London (AFP) - Billy Vunipola faces a race against time to be fit for the Rugby World Cup after undergoing a minor operation on his injured knee.

The England forward remains with a 36-player squad named by head coach Steve Borthwick for their third World Cup training camp in south-west London.

“The medical team are positive about my prospects and I understand exactly what needs to be done over the next few weeks,” said Vunipola.

Will Joseph and Beno Obano are the only two players cut from the squad by Borthwick as players from Premiership finalists Sale and Saracens are still not included.

“I’m pleased with the progress that’s been made over the last two weeks and we look forward to another productive training camp,” said Borthwick.

“There’s still a long way to go until we are ready for a World Cup but each day we take another step forward.”

Ollie Chessum, Luke Cowan-Dickie, Ollie Lawrence and Jack Walker all remain with their teammates as they continue their rehabilitation from injury.

England will take on Wales in Cardiff on August 5, with Borthwick due to name his 33-strong squad for the World Cup in France two days later.