FIFA has threatened to ban any player or club that participates in breakaway competitions.
This comes amid renewed talk of a European Super League, which will involve some of the world’s biggest teams.
A breakaway continental tournament has been mooted and discussed for years, with Barcelona, Real Madrid and many top Premier League clubs believed to favour a ring-fenced European league.
However, this idea has been vehemently rejected by others, including Bundesliga giants Bayern Munich.
UEFA President Aleksander Ceferin and LaLiga chief Javier Tebas are also well-known critics of the breakaway plans.
Now the world football governing body has made its stance clear, saying that players will be banned from playing in any of their competitions, including the World Cup and European Championship, should they take part in any unauthorised events.
The letter has been signed by the six heads of FIFA’s continental confederations and reads in part: “In light of recent media speculation about the creation of a closed European ‘Super League’ by some European clubs, FIFA and the six confederations once again would like to reiterate and strongly emphasise that such a competition would not be recognised by either FIFA or the respective confederation.
“Any club or player involved in such a competition would as a consequence not be allowed to participate in any competition organised by FIFA or their respective confederation.
“As per the FIFA and confederations statutes, all competitions should be organised or recognised by the relevant body at their respective level, by FIFA at the global level and by the confederations at the continental level.”