The Rugby Football Union has put pressure on the Guinness Premiership clubs by questioning the legality of the Long Form Agreement. The RFU and Premier Rugby Limited are in dispute over the Agreement which covers a rest period for England players involved in the Lions tour.
PRL has said the agreement applies to England games but not to the Lions.
But if PRL says it is not binding, the RFU may withhold �1m in payments for players called up under the agreement.
The RFU maintains the 11-week stipulation contained within the LFA does apply to Lions matches and has withheld �120,000 in payments from three clubs that they claim broke the agreement.
The PRL's lawyers responded by issuing a high-court writ to reclaim the money but the RFU's legal advice is that the clubs were in breach of the LFA.
"There is no clarity from the PRL if they consider the LFA to be legally binding," said an RFU spokesman.
"Today a letter was sent to the PRL. It was a short letter asking whether they view the LFA as legally binding.
"All we need is a yes or no answer - is this agreement binding?
"The RFU have requested an answer by Monday and if it is yes then the money goes."
RFU chief executive Francis Baron is anxious for the dispute, which centres on England training days and the management of elite players, to be settled by the end of November.
He has warned that England's chances of defending the World Cup in two years' time will be irreparably damaged unless a solid agreement which carries through to France 2007 can be set in place.
If PRL accept the LFA they would in effect be confirming their agreement to the introduction of "tripartite player contracts" which would give England priority during certain blocks of the season.
The provision for tripartite contracts - to be held by club, player and country - already exists in the LFA but has not been implemented.
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The RFU have requested an answer by Monday and if it is yes then the money goes