Fulham face FIFA action over their deal for 18-year-old midfielder Ange-Freddy Plumain.
The Mail on Sunday says French Ligue 2 side Lens were furious when Fulham confirmed the transfer on their website and the club released a statement claiming Plumain was still under contract and Martin Jol's side had therefore broken FIFA rules.
"English club Fulham recently reported the signing of Ange-Freddy Plumain," read an official statement from Lens.
"Racing Club de Lens is surprised to hear the information to the extent that the player still under contract, and had no authorization on a possible engagement.
"The player has been absent from his place of work since July 8, without a valid reason. A procedure has been initiated against the various parties concerned."