England have arranged an extra autumn Test against New Zealand at Twickenham on 5 November despite no reassurances their top players will be available.
Coach Andy Robinson might have to field an under-strength team
The match, to celebrate the opening of Twickenham's new £100m South Stand, is outside the designated Test 'window'.
England are already playing Argentina and South Africa, twice, in November.
Premier Rugby, the body representing England's top clubs, opposes the game and will be "writing to the RFU before seeking a court declaration".
It claims the game breaks an agreement with the Rugby Football Union about the number of matches in a season.
"The clubs believe their players are not required to be released," a statement from Premier Rugby read.
"Indeed, the Rugby Football Union itself has previously confirmed it cannot require release outside the international window."
We want Premier Rugby positively involved, not sniping on the sidelines
RFU chief executive
But RFU chief Francis Baron insists there is no danger of the game being cancelled or of England fielding an under-strength team.
Baron claims the match does fall within the 2001 Long Form Agreement (LFA) about player release.
The RFU has offered a £1m compensation package to the clubs - a figure Premier Rugby disputes - plus additional compensation to those clubs affected by the game being staged on a Sunday.
You want to take on the best and, at the moment, that's New Zealand
Although England will now play 11 Tests in 2006, Baron says the players will not be expected to play more than the stipulated 10.
He also insisted no player would be involved in all four autumn Tests - meaning a second-string side is likely to be employed against Argentina.
"We've made proposals to Premier Rugby and hope to reach accommodation with them," Baron said.
"We want them positively involved and to not sit sniping on the sidelines.
"We have removed all the issues we think have been of concern to Premier Rugby.
"Premier Rugby under the Long Form Agreement have to release players for all international matches at all levels.
"We guarantee the availability of all England players to play in club matches is not affected by this additional match.
"There is not an issue there. I can't see this game being under threat. We have made a good and substantial offer. I believe that ultimately common sense will prevail."
The Test against the All Blacks will be screened live on BBC1.
England head coach Andy Robinson confirmed that no player would start more than three of the four autumn internationals.
"As a player and a coach you want to take on the best and, at the moment, that's New Zealand," Robinson said.
"These four matches will be a huge test for us but one we will relish."