In a statement released Sunday, UEFA and six European football confederations said any teams involved in setting up a breakaway, closed 'Super League' would be barred from entering other competitions.
A group of elite 12 elite English, Spanish and Italian clubs dramatically split European football on Sunday by announcing the formation of a largely-closed Super League (ESL).
They are leaving the existing UEFA-run Champions League structure despite warnings they could be kicked out of their domestic competitions and face legal action.
The seismic move to shake up the world's biggest sport is partly engineered by the American owners of Arsenal, Liverpool and Manchester United who also run U.S. franchises in closed leagues — a model they are trying to replicate in Europe.
The Super League's 12 Founding Clubs are AC Milan, Arsenal, Atletico Madrid, Chelsea, Barcelona, Inter Milan, Juventus, Liverpool, Manchester City, Manchester United, Real Madrid and Tottenham Hotspur.
In a statement released on Sunday evening they said they expect three more clubs to join ahead of the inaugural season, "which is intended to commence as soon as practicable."
Their announcement was swiftly condemned as a "cynical project" by the European Football Associations (UEFA), the English Football Association, the Premier League, the Royal Spanish Football Federation (RFEF), LaLiga, the Italian Football Federation (FIGC) and the Italian Lega Serie A.
In a joint statement they wrote that the idea"founded on the self-interest of a few clubs at a time when society needs solidarity more than ever.
"We will consider all measures available to us, at all levels, both judicial and sporting in order to prevent this happening. Football is based on open competitions and sporting merit; it cannot be any other way."
The statement also reiterated a past assertion that clubs involved in setting up a so-called 'Super League' would be banned from playing in any other competition at domestic, European or world level.
The European Club Association, which represents the interests of professional association football clubs in UEFA, also said on Sunday it would "strongly oppose" any such plans.
How would the European Super League work?
The 'Super League', a proposed 20-team annual competition, would see 15 top European clubs become permanent members, based on plans revealed earlier this year.
The remaining five teams would vary each season, although the qualification method has not been determined.
Each of the 15 founding members would get a share of at least €3.5bn in initial infrastructure grants. The money would be split among four tiers of clubs, with the top six each getting €350m.
In their statement, the funding clubs said that dialogue with UEFA to reform the current European competition format did not solve fundamental issues, "including the need to provide higher-quality matches and additional financial resources for the overall football pyramid."
"The formation of the Super League comes at a time when the global pandemic has accelerated the instability in the existing European football economic model.
"The pandemic has shown that a strategic vision and a sustainable commercial approach are required to enhance value and support for the benefit of the entire European football pyramid," they added.
Florentino Perez, Real Madrid president and the first chairman of the Super League said: "We will help football at every level and take it to its rightful place in the world. Football is the only global sport in the world with more than four billion fans and our responsibility as big clubs is to respond to their desires."
The creation of the ESL has been condemned by British Prime Minister Bois Johnson as "very damaging for football".
"The clubs involved must answer to their fans and the wider footballing community before taking any further steps," he added.
The office of French President Emmanuel Macron said in a statement to Reuters that he "welcomes the position of French clubs to refuse to participate to a European football Super League project that threatens the principle of solidarity and sporting merit."
Fans' associations have also reacted negatively.
The Arsenal Supporters' Trust described the ESL as a plot "forcing more meaningless games on fans" and as "nothing more than crisis profiteering."
The Chelsea Supporters' Trust branded the announcement "the ultimate betrayal."
"This is a decision of greed to line the pockets of those at the top and it has been made with no consideration for the loyal supporters, our history, our future or the future of football in this country," it added.
Liverpool's Spirit of Shankly's fan association announced they "are appalled and completely oppose this decision".
The Tottenham Hotspur Supporters' Trust warned the Board of the club against what it described as an "opportunistic pursuit of greed."
"We demand the Board immediately disassociates itself from the breakaway league. Only then can meaningful discussions about change take place. If the Board does not do this, we will have no choice but to call on new owners prepared to safeguard the past, present ad future of our great Club to step forward and work with us," it wrote.