He's been developed by London Wasps, despite a period when he struggled with illness and injury
Wasps boss Dai Young has expressed his disappointment at loose forward Billy Vunipola's decision to join Saracens at the end of the season.
Vunipola has made three tournament appearances for Wasps, scoring a try against the Dragons, but has opted to leave the London club at the end of the season.
"We are clearly hugely disappointed," said Young.
"Billy is a player that the club identified some time ago as one with huge potential and he's been developed by London Wasps, despite a period when he struggled with illness and injury, into one of the brightest young talents in English rugby.
"Although we are bitterly disappointed right now, it is only fair for me to say that I believe that Billy will become a top quality international player for years to come and we wish him the best with his career.
"There is a long way to go in the season and I am sure he will give his all for Wasps in the coming months."