Regional Rugby Wales, the umbrella organisation which represents Cardiff Blues, the Ospreys, Newport Gwent Dragons and the Scarlets, said it favoured competing in the proposed Rugby Champions Cup from next season over the Heineken Cup.
The move is in defiance of the Welsh Rugby Union, which opposes the new tournament, and is a blow to European Rugby Cup (ERC), which runs the current two-tier system but has come under pressure from Premiership and Top14 clubs.
The ERC has a meeting scheduled today in Dublin though that, and the appointment of an independent mediator, look increasingly futile.
“Whilst there remain elements of detail to be confirmed, it is clear there are a number of significant advantages to the new competitions,” said Regional Rugby Wales.
English and French clubs favour a new tournament under which they receive an equal share of revenue and a greater number of qualifying places.