Aviva Premiership Rugby club London Wasps have been left furious after No.8 Billy Vunipola announced that he was leaving the club at the end of the season.

Vunipola is understood to have told London Wasps management as well as his team mates that he was staying at the club.

However London Wasps were shocked to find that the prop had signed with Premiership rivals Saracens on a long term contract.

Saracens announced on their website that a dozen players had committed to the North London club and listed 10 current players who had extended their contracts.

The former Premiership champions then added, "In addition James Johnston, tighthead prop, and Billy Vunipola, the No.8, have both signed long-term contracts with Saracens."

London Wasps responded with a statement on their website saying, "The club has been informed this afternoon that Billy Vunipola has decided to leave at the end of the season.

"We are incredibly disappointed that he has chosen to leave and we will make a full comment on his departure tomorrow."

Vunipola is reported to have met with London Wasps and both parties verbally agreed to continue their relationship with Wasps giving Vunipola exactly what he had asked for.

However it has now been revealed that Wasps only found out about Vunipola's signing half an hour before it was announced on Saracens website and even though he and his agent met with Wasps about a contract renewal he had already signed with Saracens last week.  

Meanwhile Harlequins are understood to be equally upset about the departure of James Johnston's to Saracens.

The 26-year-old prop forward joined the London club from Richmond in 2009 and has since made 88 appearances, including the Aviva Premiership final last May.

"We are very disappointed to lose a player of James' ability," said Conor O'Shea, Harlequins Director of Rugby.

"He has developed greatly since he joined us four years ago and I'm sure he is destined for great things in the game."

"Unfortunately, given the constraints of the salary cap, we were unable to meet his financial expectations so he has decided to move on at the end of the current season but I know he will give his all to go out on a high from the club that has become his extended family and means a great deal to him."