IRB will not sanction cross-border competition without union consent
The International Rugby Board remains concerned with the continued wrangling over the future of European rugby.
It comes ahead of a Welsh Rugby Union meeting regarding a possible break away by its regions.
With English clubs pulling out of the Heineken Cup, a new Anglo-Welsh competition has been suggested.
Chairman Bernard Lapasset said: "The IRB will not support any cross-border competitions that are not approved by the unions of any participating clubs."
Lapasset also reiterated there had to be consent from rugby bodies and host countries.
Although the IRB's stance has been consistent throughout, the timing is significant because it could stop any prospect of a rebellion by the Welsh regions, who are in dispute with the WRU over funding, player contracts and new competitions.
The four regions - Cardiff Blues, Newport Gwent Dragons, Ospreys and Scarlets - failed to meet a 31 December deadline to reach a new legal agreement over the game's future.
It is the latest part of a long-running battle after top-flight English and French clubs said they are to set up a new tournament to replace the Heineken Cup from next season.
The clubs gave notice in June 2012 of their intention to pull out of the two existing European competitions at the end of 2013-14.
But France's leading sides changed their stance in November and aborted the tournament they had planned with English clubs as a replacement for the Heineken Cup.
The Rugby Football Union has welcomed the IRB's latest statement, with chairman Bill Beaumont confirming urgent work continues towards a solution.
Beaumont said: "Our primary focus should be to maintain a genuinely pan-European tournament. Rest assured I am working closely with RFU chief executive officer Ian Ritchie and other stakeholders to find a solution.
"I welcome the support of the IRB in helping to ensure that this happens and, of course, as a member union abide by and support the IRB regulations."