England are willing to risk starting Billy Vunipola throughout the build-up to the World Cup in the belief that he is at his most effective when playing regularly.

Vunipola is the only member of Eddie Jones' 31-man squad who will have started all four Tests this summer after the Saracens number eight was retained for Friday night's send-off against Italy in Newcastle.

Given the extensive injury list that has scarred his recent career, continually sending one of Jones' most influential players into action during a series of warm-up games could be viewed as a gamble.

Vunipola and his older brother Mako, who is recovering from a hamstring injury, are the main source of ball-carrying momentum and their fitness is key to English hopes of lifting the Webb Ellis Trophy.

Forwards coach Steve Borthwick insists that banking as many match minutes as possible is crucial for the 26-year-old of Tongan heritage, who broke his arm three times in 2018.

"Billy absolutely wants to play. He's a player who thrives on playing – that is how you get the best out of him," Borthwick said.

"He loves playing the game and doesn't want to miss any. Look at the back row – we have had some knocks and bruises. It is about what is right for each player."

Vunipola's Saracens team-mate Owen Farrell, who will lead England out at St James' Park, shares Borthwick's view.

"Billy has been playing really well and is trying to keep it going. He enjoys getting the ball in his hands," Farrell said.

"It's important for him to be playing in the big games in which we have been involved over the last four weeks. He has got a big smile on his face because of it."

Vunipola made his own feelings clear after Ireland had been dismantled at Twickenham on August 24.

"I'm more than happy to keep going. After all my injuries and time away from the game, I'm just glad to be playing at close to the level I was operating at before," Vunipola said.

"It is all about keeping on building my fitness. If I can be fit and strong, I can help the team."

Farrell's own presence in the final Test before the squad departs for Japan on Sunday could also be considered risky given the captain, goalkicker, talisman and general is another player England dare not lose.

But in his own mind, there was never any doubt that he should start a second successive match.

"Not at all. I am excited to get back out there and looking forward to what will be a tough test," Farrell said.

"The way the squad has been building is brilliant and the exciting thing is the responsibility of keeping that momentum going."

England have eased concerns over the fitness of Henry Slade and Jonathan Joseph after summoning Joe Marchant from outside their World Cup squad to play against Italy.

The Harlequins back was omitted from the 31 selected for the World Cup but, with Slade and Joseph receiving treatment, head coach Jones has opted against exposing Manu Tuilagi to the risk of injury.

Instead, in comes Marchant who has been training with England for much of the summer and has been a part of the second camp in Treviso, north-east Italy.

It was hoped that Slade would make his comeback from a knee problem against Italy, having missed the previous three games of the summer, while Joseph was withdrawn from the rout of Ireland because of a leg muscle issue.

"Henry is doing really well. He was running (on Tuesday)," Borthwick said.

"It's just a case of that next stage, that last stage. He's just not quite ready for this game but he's close.

"Jonathan is also close. He'll be in full training (on Thursday). He's not quite ready for this game. A muscle in his leg has needed some load management."