Football's world governing body, FIFA, has been warned it is prepared to strike if players continue to be overworked.
Maheta Molango, chief executive of the Professional Footballers' Association, believes players are now at breaking point.
The PFA is calling for action to change football's congested calendar, particularly with the Club World Cup proposed to be expanded to 32 teams next summer.
“It's not even 10 days yet, but I went into the dressing rooms of the players directly affected and said: 'I'm happy to come in and bark a bit, but ultimately it's up to you, how far do you want to go?'” Molango said during an end-of-season review of players' workload and recovery conducted by the PFA and the World Players' Association's International Federation of Professional Footballers.
“Some of them said: 'I can't take it, we should go on strike.' Others said: 'What's the point? I'm a billionaire, but I don't even have time to spend it.'”
“It is not the union that is saying this, it is Jurgen Klopp and Pep Guardiola. We have reached a stage where we cannot take any action lightly.”
The PFA has threatened legal action and is considering its options for taking legal action as the number of football matches increases.
In May, in conjunction with the PFA, FIFA Pro and the World League Association (WLA), which includes the Premier League, wrote to FIFA threatening legal action if it continued with plans to stage a 32-team Club World Cup at the end of next season.
In a letter to FIFA president Gianni Infantino and secretary-general Mattias Grafström, seen by BBC Sport, FIFA Pro and the World Leagues Association said that if FIFA were to extend its own competitions, other competitions should also be changed.
“Putting the burden of adaptation on national leagues and players is inherently abusive,” it added, “putting national leagues at risk and impacting the health and wellbeing of players.”
But BBC Sport understands FIFA believes it has taken players' welfare into account in its scheduling plans.
Molango added: “Some of the changes to the English domestic league schedule have been forced upon us by action from FIFA and UEFA. This incident is further evidence that something needs to be done, but it also shows that this is not just affecting the top players.”
“We will always use all diplomatic means. We have sent letters and received responses, but unfortunately we don't have the time.
“When it comes to adults, even if you try your best to find a solution, sometimes a third party, such as an arbitrator or a court, needs to make the decision.”
Studies focusing on young players have warned about how much stress their bodies are under, highlighting the amount of time Real Madrid and England's Jude Bellingham played before turning 21.
Still just 20 years old, the midfielder has played 18,486 minutes in his career so far, compared to 3,929 for David Beckham and 6,987 for Frank Lampard, both at the same age.
FIFpro Europe chairman David Terry also warned that players were feeling physically and mentally exhausted.
In a survey of players conducted as part of a season review, more than 50% of respondents said they had played through an injury.
“It's an emergency. We are in danger,” Terrier said. “The players are overstretched and the international schedule is full.”
Infantino spoke to member associations about football's schedule issues at the FIFA Congress earlier this month.
“FIFA organises around 1 per cent of the world's top club matches. 98-99 per cent of matches are organised by the various leagues, associations and confederations,” he said.
“FIFA funds football all over the world. The revenue we generate doesn't just go to a few clubs in one country, it goes to 211 countries around the world.”
“Our mission is to organise events and competitions and to develop football all over the world, because without direct funding from FIFA, football would not be viable in 70 per cent of FIFA's member associations.”
