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.
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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, 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."
Meanwhile, UEFA's Head of Women's football Nadine Kessler said a breakaway European Super League would be detrimental to the women's game.
"The news comes without any consultation and at a time when the UEFA Women's Champions League is about to see a transformative change as of next summer," Kessler said in an open letter published on Twitter.
"In other words it is a direct threat to all the plans we have carefully crafted, together with the ECA, your clubs and the leagues for the new Women's Champions League.
"Such a development would be equally devastating for national leagues who have made enormous efforts to professionalise the women's game.
"All the great steps made in recent years, including the hardship of many players gone before for our game to become a profession, will have less chance of becoming a reality.
"No matter how you make up your mind, your opinion as a player, as a women's football champion matters. The value of our sport at a time when greed seems to overshadow the broader needs of society and football as a whole. Solidarity matters!"