(Reuters) -France's players' union has thrown its weight behind a mushrooming legal challenge to FIFA's transfer system, joining a Dutch-led class action that could involve 100,000 professional footballers seeking compensation over alleged lost income.

The French National Union of Professional Footballers (UNFP) said on Thursday its board unanimously decided to become a co-plaintiff in the "Justice for Players" (JfP) lawsuit, which stems from a European court ruling that found some FIFA transfer rules violate EU law.