The Welsh yesterday threw out Wales lock Gareth Llewellyn's request to transfer from Neath to Harlequins, and was immediately told it will almost certainly face a legal challenge from the London club.
Llewellyn has signed to join Quins next season but the WRU maintains he was not resident in England on 1 March and therefore does not meet the International Rugby Football Board residency qualifications for players seeking to play under a different Union.
It means Vernon Pugh and his committee could soon find themselves fighting on two legal fronts, as their senior clubs have already threatened to sue their governing body in their protracted battle over European Cup funding and next season's domestic programme.
While Llewellyn was restricting himself to a diplomatic "I am preparing for a very important international, I will comment next week," the WRU can brace themselves for a major test-case battle.
The WRU stresses it is merely sticking to the IRFB regulations, though it did add the rider that "the decision was taken with the aim of preserving the strength of domestic Welsh rugby''.
Tony Hallett, the RFU secretary, said: "The RFU will stand by Harlequins. The RFU's position is that it is mindful of international board regulations, but it is equally mindful of the Treaty of Rome and restraint of trade. Which has the greater authority? It has to be the law."
Dick Best, Harlequins' director of rugby, said: "If necessary, we will issue a writ against the Welsh . We envisaged something like this would happen.''Reuse content