World soccer's governing body FIFA says players who feature in any breakaway European Super League would be banned from playing in FIFA competitions, including the World Cup.
In a joint statement with European governing body UEFA and the other five continental confederations on Thursday, FIFA said they would not recognise any such breakaway.
FIFA says they have all acted "in light of recent media speculation about the creation of a closed European "Super League" by some European clubs."
The statement said FIFA and the six confederations "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."
The idea of a breakaway league, as an alternative to UEFA's flagship Champions League, has been floated for many years but speculation has intensified in the past two years.
The timing and joint nature of the declaration indicates a growing concern that the threats of a breakaway could be more than just a negotiating tactic from the big clubs.
In November 2018, German magazine Der Spiegel, citing leaked documents, reported that Real Madrid had been making plans for a Super League to feature the continent's biggest clubs.
Although a number of those clubs denied they were in talks, Real president Florentino Perez has continued to talk of the need for major changes in European club competition.
In October, outgoing Barcelona president Josep Maria Bartomeu said he had signed the club up to a Super League.
Thursday's joint statement also gave clear backing from UEFA and the other confederations to the plans for an expanded FIFA Club World Cup.
UEFA had been cautious about the plans for an expanded 24-team tournament.
Australian Associated Press