Richard Hill had an exploratory operation on his left knee last night amid fears that he may miss not just England's friendly internationals next month but also the start of the Six Nations Championship.

The 31-year old Hill, the only member of England's World Cup-winning back row unit still playing international rugby, twisted his knee making a tackle during Saracens' league defeat at London Irish last Sunday and left the ground on crutches. He had a scan on Tuesday after which it was decided he needed an arthroscopy to assess whether he had suffered significant ligament damage.

"I thought I had made progress over the last couple of days and to be told I needed the exploratory operation was not the outcome I had been hoping for," said the 71-cap Hill. "The arthroscopy will determine the extent of the injury."

Hill was identified by the acting England head coach Andy Robinson as the pack leader for the Twickenham internationals next month against Canada, South Africa and Australia, but if he is ruled out it would mean half the pack that started the World Cup final against the Wallabies less than 11 months ago were unavailable to him. Martin Johnson, Lawrence Dallaglio and Neil Back have retired from international rugby and the props Trevor Woodman and Phil Vickery are this month making their comebacks from injury.

Robinson has no injury worries over the other two forwards on duty that day, the hooker Steve Thompson and the lock Ben Kay, but neither has recaptured the form they were in last year, with Kay used on rota by his club Leicester.

Hill, who led England last December against the New Zealand Barbarians, was named by Robinson as one of two players, along with Mike Tindall, who would help the new captain Jonny Wilkinson by assuming leadership duties in training sessions. With Robinson already looking for a No8 to replace Dallaglio, the loss of the experienced and versatile Hill, who has played in all three positions in the back row, would leave him with a considerable problem.

The former England head coach Sir Clive Woodward will be monitoring Hill's injury as he prepares for next year's Lions tour to New Zealand, a trip the Wales and Wasps scrum-half Rob Howley aims to make. Howley has not played since last May's Premiership final after having an operation on a broken bone in his wrist. Yesterday he admitted he was facing up to the prospect of not playing at all this season because the fracture has not set.