Dublin - European rugby could be on course for civil war after the Scottish, Welsh, Irish, French and Italian unions on Thursday announced their commitment to existing European club competitions after a meeting in Dublin.
The unions' determination to push on with a competition under European Club Rugby (ERC) control risks isolating English clubs who, along with their French counterparts, are planning a breakaway contest to replace the Heineken Cup from next season.
The English Premiership responded by saying those plans remain on track, leaving the landscape of European rugby next season - almost two decades after the introduction of the Heineken Cup - still a long way from being settled.
"All five unions believe that it is critical to the interests of the game in Europe that the unions are at the heart of the governance of cross-border club competitions," the unions said in a joint statement, with England's Rugby Football Union not involved.
"Cross-border club competitions must not conflict with the development of the sport in Europe by Unions, this being in the best interest of players, spectators and the sport in general."
They added that 20 teams, a reduction from the current 24, would take part in next season's European club competition "no matter how many countries are involved."
Ireland, whose sides have excelled in the Heineken Cup in recent years, have been holding out along with the Scottish and Italian Unions in the hope that negotiations can save the existing competition.
The public support of the French Union is a major boost. French Federation president Pierre Camou told the meeting he was confident five Top 14 clubs would remain in the competition and that he was looking for three more to join the 12 Celtic and Italian teams, the Guardian newspaper said on its website.
The Welsh Union's stance also puts it at odds with the four Welsh regional teams - Cardiff Blues, Ospreys, Scarlets and Newport Gwent Dragons - who last month surprisingly backed the Anglo-French clubs plan.
The English and French leagues said last year they planned to withdraw from the existing format at the end of this season after making no headway in negotiations with the Celtic unions over qualification criteria and income distribution.
Disagreements about the tournament's future TV rights - with newcomers BT coming into conflict with long-standing Heineken partner Sky - has also proved a major stumbling block.
The head of the English Premiership told Reuters last week that the new club competition - the two-tier 'Rugby Champions Cup' - is a "train that has left the station" and it was just a question of which teams wanted to join.
In their own statement on Thursday, English Premiership said: "There is no detail concerning the teams involved or the competition format given the absence of so many teams.
"We shall continue to implement the plans underway for the Rugby Champions Cup with the declared participants in time for the 2014-15 season, as required by our clubs."