The Rugby Football Union (RFU) have written to Premier Rugby and the chairmen of all 12 Guinness Premiership clubs highlighting the stark legal implications of their proposed Heineken Cup boycott.

The leading clubs in England and France voted not to participate next season in a row with the RFU over shareholding and voting rights of European Rugby Cup Ltd.

But Twickenham's top brass warned the clubs that failing to play in the Heineken Cup would invalidate the Long Form Agreement, the blueprint for professional rugby in England which runs until June 2009.

RFU chief executive Francis Baron also stressed the clubs would be in breach of contractual obligations with sponsors Heineken and broadcasters Sky, leaving them open to potentially expensive litigation.

Baron said: "By the action the clubs are proposing to take, it does put them in breach of clause 5.1 of the Long Form Agreement.

"More importantly, the whole of the Long Form Agreement is actually conditional upon the participation of the clubs in Europe.

"If the clubs do not participate in the Heineken Cup then the Long Form Agreement itself could fall away for the balance of the two years outstanding.

"The third area of legal concern we have is that PRL did approve the new Heineken deal for ERC, which goes through until 2009.

"Clearly Heineken extracted certain undertakings from the participant clubs prior to signing that agreement. Heineken believe they have a clear agreement approved by Premier Rugby going through to 2009.

"With the Sky deal, similar provisions apply. Premier Rugby approved the ERC Sky deal through to 2010. Sky believe they have the commitment of Premier Rugby to compete.

"If the clubs do not compete in the Heineken Cup then both Heineken and Sky have legal and financial redress with regards to those contracts.

"And finally, the RFU will have to consider whether the conduct that has resulted to the breach of the various agreements and the non-participation in Europe is conduct prejudicial to the interests of the union and the game."

The RFU letter requests the clubs reconsider their decision and Baron insists the union are not being confrontational.

"The key thing here is that we do not wish to see this matter proceed to litigation in any way, shape or form," Baron said. "This matter has to be settled within the rugby family with common sense prevailing.

"We want to see the temperature drop over the next few days because until then I don't think rational discussion will take place. However, we have had to put in writing to Premier Rugby and the clubs what the position is just so everyone is aware of it."

The six members unions of ERC agreed last week that next season's Heineken Cup competition will go ahead, with or without the top-flight teams from England and France.

Both Heineken and Sky are expected to stick with the tournament, in whatever guise it is played next season, but are known to be watching developments closely.

They have the right to terminate, or renegotiate their deal or even seek damages if they believe an agreement has been breached.

Without the Heineken Cup, English clubs could lose in the region of £1million each and if the sponsors and broadcasters decide to reduce their input the shockwaves could be felt across the European game.

England's elite rugby director Rob Andrew said: "The financial implications are very stark. You are talking millions and millions of pounds across the European game.

"If a Premiership club will be in excess of £1million short next year that will have serious implications."

Even if the tournament goes ahead at full strength, Baron believes long-term damage has been done.

"After the English clubs boycott of 1998 it took three years to build the revenue back up again," he said.

The five-year ERC agreement put in place confirms the English representatives in European competition will be clubs rather than regions.

That measure should ease some concerns within Premier Rugby that the RFU are looking to introduce centrally-run franchises to replace the current club system.

But attempts to find a new workable solution to the entire club versus country issue in England has hit stony ground.

Baron and RFU management board chairman Martyn Thomas have not spoken with Premier Rugby negotiating chief Tom Walkinshaw in over a month.

The RFU have accused Premier Rugby of going back on "good faith" talks just as they appeared to have found a breakthrough.

The RFU say they were offered a proposal whereby the management of England's elite players would be handed over to Andrew in return for increased funding.

But that deal is now off the table and Andrew admitted: "It is very frustrating because from a negotiating point of view we are back to square one.

"We only went down the road of central management because it was offered to us."

The RFU dismissed the idea of resorting to conciliation service ACAS or another third party in a bid to thrash out a solution.

But they do believe the players' union should be allowed into the negotiations, and Baron was meeting Professional Rugby Players' Association chiefs Damian Hopley and David Barnes today.

Final confirmation of PRA's involvement at the top table is required from Premier Rugby.