Wasps officials were left shocked and angry on Monday night after Saracens signed their number eight Billy Vunipola on a "long-tern contract".
Saracens have also signed Harlequins prop James Johnston in a double transfer coup for the Aviva Premiership club - but there is bad feeling betwen the London rivals.
Wasps had been expecting Vunipola to remain with them, believing that at a meeting today with the player and his agent would see a new contract being discussed, with Wasps prepared to give the 20-year-old exactly what he asked for.
But it is understood Vunipola actually signed his deal with Saracens last week. Wasps say they discovered he was leaving just half an hour before Saracens made the formal announcement.
A Wasps statement said: "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 one of the most exciting young players in the Aviva Premiership and he has been fast-tracked into England's RBS 6 Nations plans.
The dynamic number eight will join brother Mako at Saracens, who is one of 12 players to have agreed contract extensions with the Men In Black.
Harlequins, it is understood, were equally as taken-aback that Johnston had decided to move across London to Saracens.
While the loss of Vunipola will hit Wasps hard just as Young looks to have finally got the club moving in the right direction, Johnston's departure is a similarly huge blow to Harlequins.
The Samoan international was the cornerstone of Harlequins' title triumph last season and he has developed over the last year into one of the most dominant tight-head props in the Premiership.
England internationals Richard Wigglesworth and David Strettle have also agreed new deals with Saracens along with Saxons squad member Will Fraser.
Eoin Sherriff, Ben Spencer, Nils Mordt, Duncan Taylor, Ben Ransom and James Short have also extended their stays at the club.
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