The Welsh Rugby Union says it will not sanction any of its regions or clubs playing in future tournaments they do not fully approve of.
It follows plans announced earlier this week for a new Rugby Champions Cup next season led by top English and French clubs.
Premiership Rugby and Ligue Nationale de Rugby say they have no interest in resuming negotiations with European Rugby Cup over a new tournament agreement for the Heineken Cup and Amlin Challenge Cup competitions. The current accord expires next summer.
Talks are not planned to resume until October 23, and ERC has appointed a mediator - Canadian lawyer Graeme Mew - in an attempt to bring all sides together.
But Premiership Rugby and LNR are forging ahead with their own plans, having also extended an invitation to Celtic and Italian teams to join them.
"The Welsh Rugby Union remains fully committed to the development of a pan-European rugby competition," the WRU said in a statement.
"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 WRU's focus is to work collaboratively with our colleagues across Europe, encouraging all parties to conclude our negotiations as quickly as possible."
The WRU's stance serves as a clear message to their four professional regions - the Ospreys, Cardiff Blues, Scarlets and Newport Gwent Dragons.
And that was quickly followed by similar statements released by the Scottish and Irish rugby unions.
"Scottish Rugby wishes to clarify that its clubs will not be participating in future tournaments which do not have the full approval of the International Rugby Board or the relevant national rugby unions," the SRU said.
"Scottish Rugby and its clubs remain fully committed to the development of a pan-European rugby competition and we welcome the recent comments made by the IRB chairman, who confirmed that a pan-European tournament remains the goal of the IRB. We are confident this can be achieved.
"Scottish Rugby remains committed to working with our colleagues across Europe and encourages all parties to fully engage in meaningful negotiations. We hope that negotiations can be concluded quickly."
The IRFU's viewpoint replicates that of both the Welsh and Scottish unions.
The French Rugby Federation, meanwhile, has already said they will not support any new Anglo-French competition.
Premiership Rugby and LNR, though, plan to finalise structural detail of the Rugby Champions Cup when it knows who will be involved.
Speaking earlier this week, Premiership Rugby chief executive Mark McCafferty said: "Before we can release more information we need to know over the next few weeks which other teams will be joining the competition.
"Once we know the final, or close to final, numbers then we'll settle on the competition formats.
"Various people have approached us about the possibility of joining and we've indicated to them that we'll keep their names confidential.
"We've heard that other people want to be involved in European competitions, that's why we've left the door open and expect it to be more than an Anglo-French competition.
"We're asking for the RFU's (Rugby Football Union's) support against the background that there will be no ERC competition in which we're taking part at the end of the season. We need to put in place new competitions."