France's top 14 clubs insisted yesterday that their boycott of next season's Heineken Cup was not an empty threat, and have called on the International Rugby Board to take action to halt the proliferation of international fixtures and set up a sub-committee which includes club officials to help run the professional game.

The French league clubs (LNR) said that unless action was taken, professional rugby in Europe would wither. They fear that the IRB wants to bring France and England, the only two major unions which operate a club system, into line with the rest of the world where control of leading players largely lies with governing bodies.

LNR believes that the Rugby Football Union's refusal to agree to proposals to give French, Italian and English clubs share rights on the board of the organisation which runs the Heineken Cup, European Rugby Cup Ltd, is proof that it intends to field franchises in Europe, rather than clubs, after 2009, while it described the IRB's international fixture schedule as outrageous.

"The IRB has to take into account the strides club rugby has made and work with us," said an LNR statement. "The board needs to set up a group made up of the various parties concerned to organise the professional game, because to do nothing will threaten everything. France could play 19 internationals this year and we are obliged to release our players for all of them, but that raises issues of employment law. The current system is harming national teams as well as clubs and the board must act."

Premier Rugby meets on Monday to consider its response to the French boycott. "LNR makes a number of salient points," said the Sale chief executive Niels de Vos. "Why are the RFU refusing to go along with everyone else on the ERC board and agree to changes which will allow the two European tournaments to be run on hard-nosed commercial lines? It would allow us to improve our finances, run bigger squads and be in a position to release England squad players more often. There may be some merit in the idea that the Heineken Cup takes a sabbatical next season because of the World Cup.

"[That] would ease fixture congestion - and why is it that the European nations are all touring in the summer of 2008 when players should be having time off? Why are England playing Saxons matches next month and denying players their only chance of a two-week break in the season? The international calendar needs to be looked at, and pushing the Six Nations back to the end of the season would be a start."

The RFU, which has had to put tickets for next month's fixture against Italy on general sale because clubs did not take up their allocation, agrees with that idea. "We have argued for it because it would allow club competitions to be played out before the Six Nations starts and there would be no issues over player release," said the union's management board chairman, Martyn Thomas. "Unfortunately, the French will not agree because they say their season has to end with their league play-offs and championship final."

The French Rugby Federation president Bernard Lapasset said he sympathised with the clubs but pointed out that France could not complain about clutter when it had 26 league fixtures and a season that ran from August until the end of June.

"French players are burning out," said the Wales and Toulouse full-back Gareth Thomas. "I see at first-hand that they are tired because they have hardly had a break in four years. The French league is already congested and the World Cup will clog it up even more. I am not surprised that they have pulled out of the Heineken Cup."

The Guinness Premiership clubs are bound to compete in Europe until the end of the 2008-09 season under the long-form agreement they have with the RFU, although that is likely to be challenged on Monday. "There are so many loopholes in that agreement that I am sure we could get out of it if we wanted to, because without the French it is not the tournament we signed up to," said one club official.

"It is important we support the French because we are fighting for the future of the club game and we cannot trust the RFU. If we lose, England will suffer."