England's Rugby Football Union have announced they would launch legal action against any English club that attempted to boycott any of next season's European competitions, such as the Heineken Cup.
Premier Rugby announced on Monday that English sides would not play in Europe next season because a planned boycott by French clubs.
But RFU chairman Martyn Thomas spelled out the consequences of a similar move.
"I hope the boycott can be averted,
"But it could be a situation where rugby has to resort to the courts - and I would deeply regret that."
In outlining the RFU's position Thomas said, "We have a contract with our clubs which we signed in 2001 and in relation to Europe it is very simple.
"It says the clubs will compete in the European Cup or the European Shield. That lasts until 2007 with the long-form agreement until 2009.
"So I am at a loss to understand their statement last night that they believe they can pull out."
The trouble began in January when France's national league (LNR) said it was pulling its clubs out of Europe because of a dispute in England between the RFU and Premier Rugby clubs, saying the row had led to uncertainty over the future of the competitions.
That dispute centres on voting rights and shares in the European Rugby Cup (ERC), with the Premier Rugby clubs wanting the RFU to give them 50 percent of its stakes and voting rights.
Announcing the English boycott on Monday, Premier Rugby, who oversee the 12 Guiness Premiership clubs said, "A European Cup without the leading French clubs is not a genuine European competition and Premier Rugby's clubs have today agreed unanimously not to participate in any such competition without the French next season."
A boycott would not affect this season's Heineken Cup, three of whose semi-finalists are English: Northampton, Leicester and Wasps.