Midfielder Hatem Ben Arfa is not allowed to play for Nice this season, after Fifa ruled a Newcastle Under-21 game counted as a competitive match.
The 27-year-old joined Nice from Newcastle earlier this month after cutting short a loan spell at Hull.
However Fifa says players can only play "official matches" for two clubs in a season and has ruled that the Under-21 Professional Development League counts.
Nice, who signed Ben Arfa on an 18-month deal, plan to take it to court.
France international Ben Arfa scored in a 5-2 win over Reading Under-21s in August before going on loan to Hull, where he played nine times.
His loan spell with the Tigers was cut short after falling out with manager Steve Bruce and he moved to Nice on a free transfer.
The French league (LFP) asked football's world governing body for clarification on the status of the Under-21 Professional Development League, where three over-aged outfield players are allowed, before making an announcement.
"In application of the decision, the LFP has no other choice but to ban Hatem Ben Arfa from playing official matches with Nice until the end of the season," a statement said.