English and French clubs are prepared to take legal action to protect their domestic and European competitions from encroachment by the new global tournament proposed by World Rugby.
The annual Nations Championship would see the Six Nations and Rugby Championship fixtures contribute towards a 12-team competition that would extend the autumn window by including play-offs and a final.
The idea has met with fierce opposition, most notably from player welfare groups, while the suggestion that the Pacific Islands nations of Tonga, Samoa and Fiji could be excluded has also provoked fury.
World Rugby chairman Bill Beaumont will host a meeting of tier-one countries, Fiji and Japan and the international players' union in Dublin on Thursday to discuss the idea.
Representatives from the Premiership and Top 14, as with the Pro14, have not been invited, however, even though they would face losing some international players for additional weekends.
In a joint statement released by the English and French clubs, they have expressed their dismay that the outline for the new global season that begins in 2020 as negotiated in San Francisco has been discarded.
It is understood they are willing to pursue the matter through legal action if necessary.
"The San Francisco agreement reached in January 2017 by all stakeholders including LNR (French Rugby League) and PRL (Premiership Rugby) represented a proportionate structure for all parties with an international calendar adopted until 2032," the statement read.
"It appears that the new competition plan now challenges this balance by increasing the number of international matches, including among other periods extending into December with four or five successive international weeks which had been specifically rejected for established annual international competitions.
"The professional leagues now seem to be excluded from this new work, even though the World Rugby project would be a major change to the San Francisco agreement for all elements of the professional game, and impact heavily on other competitions.
"LNR and PRL regret that World Rugby is not fully involving all stakeholders in seeking a consensus and they can only reserve the option to take any action to preserve their rights and competitions."