While four clubs have agreed to sign up, Cardiff and Ebbw Vale went to the High Court in London yesterday to start legal proceedings which were adjourned until a full hearing on 26 June.
The WRU said: "The Union considers the application for injunctions to be misconceived and they will be vigorously contested. However, between now and the June hearing the WRU has agreed that it will take no steps to expel the clubs or to nominate any club outside the Premier Division to participate in the Heineken Cup or European Conference."
Cardiff insisted that their action was in the best interests of the game in Wales. In a statement, the club said: "It is not a step we have taken lightly. We were forced to do so in order to protect the very existence of Cardiff RFC and the interests of our employees, players, shareholders and supporters."