A strike by top soccer stars cannot be ruled out, a spokesperson for the sport’s global players’ union said Wednesday, as the newly expanded Club World Cup continues to face pushback.

The FIFA-run competition has been expanded from seven teams to 32 and will be staged in the United States from June to July next year.

Its new format has faced a fierce backlash , with players’ union FIFPRO raising concerns about the physical and mental impact on players as a result of an increasingly congested schedule.

“We have seen players’ strikes in different sports. We also have seen it in professional football and the players (have) proactively talked about it,” Alexander Bielefeld, FIFPRO director of policy and strategic relations, said. “I think it’s quite a unique moment in terms of how connected players are on this issue. So I, personally, would also not rule anything out at this point.”

Manchester City midfielder Rodri said in September that players were close to taking strike action because of the number of games they are having to play. He sustained a likely season-ending injury days later when damaging knee ligaments.

On Wednesday a report by Belgian university KU Leuven, which was commissioned by FIFPRO, studied occupational safety and health standards in relation to soccer.

FIFPRO said governing bodies had shown “no commitment to identify holistic health and safety standards to address player needs in a high-risk environment.”

While Europe’s top clubs have welcomed the new Club World Cup, it is the subject of a formal complaint to the European Commission in Brussels by players’ unions and domestic leagues about how it was added to the global soccer calendar.

FIFPRO said the report would be added to evidence for its complaint.

Despite resistance FIFA is pushing ahead with the Club World Cup, which president Gianni Infantino described last week as the “start of a new era for football.”