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.
"This matter has to be settled within the rugby family with common sense prevailing," Baron said.
"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."