FIFPRO, the global union representing football players, has launched a legal claim against FIFA over the match calendar and the scheduling of next year's expanded Club World Cup.
FIFA's decision to stage a 32-team Club World Cup in the United States next year has attracted fierce criticism from pundits and players, with many believing it will exacerbate issues arising from fixture congestion.
Earlier this week, Real Madrid coach Carlo Ancelotti suggested the European champions could boycott the tournament, only for the club to quickly confirm their participation in a statement.
Ahead of last year's seven-team Club World Cup, Manchester City midfielder Bernardo Silva said the expansion of the football calendar would lead to matches losing "energy and intensity".
When FIFA's Club World Cup plans were finalised last year, FIFPRO threatened legal action and said world football's governing body had demonstrated "a lack of consideration for the mental and physical health of players, as well as a disregard for their personal and family lives."
FIFPRO has now launched a claim against FIFA at the Brussels court of commerce.
In a statement released on Thursday, the union said: "FIFPRO Europe member unions have today submitted a legal claim against FIFA, challenging the legality of FIFA's decisions to unilaterally set the international match calendar and, in particular, the decision to create and schedule the FIFA Club World Cup 2025.
"Player unions believe that these decisions violate the rights of players and their unions under the EU Charter of Fundamental Rights while also potentially violating EU competition law."
Madrid and City are among 12 European clubs to have booked a spot at the first 32-team Club World Cup, due to their recent Champions League successes.
Chelsea, Bayern Munich, Paris Saint-Germain, Inter and Juventus will also feature, with most qualifiers being determined by a FIFA ranking system that awards points for victories in the Champions League.