The Welsh, Irish and Scottish Rugby Unions have confirmed they will not sanction any regions or clubs playing in future tournaments they do not approve of.
Invites to the new 'Rugby Champions Cup', a breakaway league led by English and French clubs, have been extended to Celtic clubs as well as those from Italy.
Premiership Rugby and Ligue Nationale de Rugby confirmed having no interest in reopening negotiations over a new tournament agreement for the Heineken Cup and Amlin Challenge Cup - the current accord expires in 2014.
They are instead forging ahead with their own plans and hoped to get backing from other federations, although the French Rugby Federation has also come out in support of the current structure being amended.
"The Welsh Rugby Union remains fully committed to the development of a pan-European rugby competition," the WRU said in a statement that was echoed by their Scottish and Irish counterparts.
"We welcome the recent comments made by the International Rugby Board chairman, who confirmed that a pan-European tournament remains the goal of the IRB.
"The Welsh Rugby Union wishes to clarify that it will not sanction any of its clubs or regions participating in future tournaments which do not have the full approval of the IRB and the WRU.
"The WRU wants an agreement to be achieved and reiterates its determination to negotiate a new format for the European Cup with all of the stakeholders. The focus is to work collaboratively with our colleagues across Europe, encouraging all parties to conclude our negotiations as quickly as possible."