Premiership Rugby and its French counterparts the LNR have been accused of showing a lack of respect during an increasingly bitter row over the Heineken Cup's future.
Top English and French clubs have announced plans for a new tournament next season, the Rugby Champions' Cup.
And Premiership Rugby chief executive Mark McCafferty has said that he expects Celtic and Italian teams to join them.
The French Rugby Federation (FFR), though, indicated it would not support such a venture.
Any Anglo-French breakaway tournament would require FFR and Rugby Football Union approval as respective governing bodies.
Canadian lawyer Graeme Mew has been brought in by European Rugby Cup to act as an independent mediator in the dispute. The next round of negotiations is planned for October 23.
But Premiership Rugby and LNR currently have no intention of being involved after failing to make headway with ERC over financial, qualification and competition structure changes that they want to the Heineken Cup and Amlin Challenge Cup.
The existing tournament accord expires next summer. English and French clubs served notice to quit the current agreement last year.
But speaking at Monday's Paris launch of this season's Heineken and Amlin competitions, ERC chairman Jean-Pierre Lux was in a combative mood.
"The FFR has now officially reminded the LNR that no international game involving French clubs can take place without the advance permission of the FFR. And the RFU has a similar position regarding Premiership Rugby," Lux said.
"The recent media releases from the LNR and Premiership Rugby lacked respect.
"In June, the LNR committed to the principle of central marketing for ERC's tournaments which has been the at the heart of European club rugby's commercial success.
"While this process began in 2012, ERC met with the parties during the summer, with the exception of Premiership Rugby, who refused to engage. They have never entered into negotiations," he added.