FIFA president Gianni Infantino says football's global governing body "strongly disapproves" of plans for a breakaway European Super League.
Infantino has warned that the clubs involved "must live with the consequences of their choice".
The plans, which have thrown the world of football into tumult, would involve 20 clubs playing in a midweek League - with the founding 12, and three others yet to join, permanent members of the competition and not subject to possible relegation.
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The 12 clubs currently involved include six from the Premier League (Arsenal, Chelsea, Liverpool, Manchester City, Manchester United and Tottenham) plus three each from Italy and Spain.
Real Madrid president Florentino Perez says there is 'less and less' interest in the current Champions League format - but the breakaway proposals have met with a chorus of disapproval and Infantino told the UEFA Congress on Tuesday that his organisation is also opposed to them.
He said: "We can only strongly disapprove the creation of the Super League, a Super League which is a closed shop, which is a breakaway from the current institutions, from the leagues, from the associations, from UEFA, and from FIFA.
"There is a lot to throw away for the short-term financial gain of some. They need to reflect, and they need to assume responsibility.
"If some elect to go their own way, then they must live with the consequences of their choice. They are responsible for their choice.
"Concretely, this means either you're in or you're out. You cannot be half in or half out."
UEFA president Aleksander Ceferin - who has strongly criticised the Super League plan - thanked Infantino for his support.
Ceferin said: "You showed that you care about the values of football.
"And if we stand together, we are unbeatable."
Ceferin pointed out that success and club size can change over time, and said: "Where were Manchester United in the decade before Sir Alex Ferguson arrived on the scene?
"And where were Juventus 15 years ago?"
Addressing the breakaway clubs directly, he added: "Come to your senses, not out of love for football, because I imagine some of you don't have much of that, but out of respect for those who bleed themselves dry so that they can go to the stadium to support the team, and want the dream to be kept alive.
"For those, you (must) change your mind."