Fifa president Gianni Infantino told the 12 breakaway European Super League clubs on Tuesday that they could not be "half in, half out" of the established football system and must face up to the reality of their decision.
Real Madrid, Barcelona, Manchester United, Liverpool and Juventus are among the members of the proposed new league, but Uefa has threatened to ban them from domestic and international competition and vowed to fight the move.
"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 are in, or you are out. You cannot be half in and half out. This has to be absolutely clear," Infantino told Uefa's congress in Montreux, Switzerland.
Infantino's comments came after the Super League had called on Monday for talks with Uefa and Fifa about their new competition's place in the football "ecosystem".
The Fifa head, a former Uefa general secretary, repeated his opposition to the breakaway project.
"We can only and strongly disapprove of a Super League which is a closed shop, a breakaway from current institutions," Infantino said.
"No doubt whatsoever of Fifa's disapproval. Full support to Uefa."
"We hope that everything will go back to normal, that everything will be settled, but always with respect, always with solidarity and with the interests of national, European and global football."
Uefa president Aleksander Ceferin thanked Infantino for his support in opposition against the European Super League.
"You showed that you care about the values of football. And if we stand together, we are unbeatable," he said.