Fifpro and the World Leagues Association believe they have already provided Fifa with ample opportunity to address their concerns.
However, in their letter they state clearly that if Fifa “refuses to formally commit to resolving the issues” at its council meeting in Bangkok on 17 May, members will be advised “on the options available to them”.
“These options included legal action against Fifa,” they said.
The Fifa council meeting in Thailand will see all 211 member associations get together, with a vote on the host nation or nations of the 2027 Women’s World Cup set to take place.
The threat of legal action against Fifa comes against a backdrop of Premier League clubs announcing lucrative post and pre-season tours.
On Thursday, West Ham, Wolves and Crystal Palace announced a three-team pre-season tour of the United States, while Newcastle and Tottenham will play one another in Melbourne, Australia just three days after the end of the Premier League season.
Manchester United, Manchester City, Chelsea, Liverpool, Arsenal, Bournemouth will all tour the United States next summer.
