Stade Francais have demanded the Rugby Football Union release England flanker James Haskell to play in a French Top 14 match against Toulouse on Saturday.
The RFU have said they are under no obligation to release Haskell during England Elite Player Squad periods, a claim denied by the Paris club.
"Contrary to what the RFU are saying, no agreement has been signed with the RFU. The RFU must be mistaking with other French clubs like Toulon who have accepted such a clause about Jonny Wilkinson," Stade said today.
"For all these reasons, we are ordering James Haskell, who has already missed two training sessions, to return most urgently."
Haskell has started all three of England's Six Nations matches so far in this year's tournament.
The RFU has a binding agreement with all English clubs over player release but has had to negotiate individually with French clubs to try to ensure any English players joining them are available.
"We are aware of the request but he is not going back," an RFU spokesman said. "We reached an agreement with Stade Francais last summer and it is in James's contract that he is available during EPS periods, so we are under no obligation to release him."
With six games remaining, Stade Francais are seventh in the Top 14, just outside the top six teams that will qualify for the end of the season's playoffs. Toulouse are fourth.