Ed Woodward has resigned as Manchester United executive vice-chairman, sources have told ESPN, after eight years in the role.
Gianni Infantino told the European Super League clubs that they could not be "half in, half out" of the established football system.
Clubs including Manchester United and Manchester City are fielding calls from players worried about the ESL implications, sources have told ESPN.
UK Prime Minister Boris Johnson said his government will do "everything we can" to stop the European Super League from going through.
The proposed Super League is not for fans, only the owners of football's biggest, richest clubs, who just want to become even bigger and richer.
UEFA's executive committee responded to the threat of a breakaway European Super League by voting to sanction the new-look 36-team Champions League.
The 12 European clubs pursuing a Super League have told the leaders FIFA and UEFA that legal action is already being pursued.
Twelve of Europe's top clubs announced on Sunday they were launching a breakaway Super League, headed by Real Madrid president Florentino Perez.
In 2012, Man United squandered an eight-point lead with six games left. A similar story is unlikely, but Ole Gunnar Solskjaer's men are progressing.
Former Manchester United manager Alex Ferguson has said a breakaway European Super League would be a move away from 70 years of football history.
A Mason Greenwood double helped Manchester United close the gap at the top with a 3-1 win over Burnley at Old Trafford on Sunday.
Paul Pogba has been linked with a move away from Man United and his former side Juventus seem ready to make it happen. Transfer Talk has the latest.
Fifteen of Europe's biggest clubs are in talks to launch a European Super League with a $6 billion fund backing the project, sources told ESPN.
Man United boss Ole Gunnar Solskjaer believes Paul Pogba will not be affected by the added spotlight of an Amazon Prime documentary on his life.
Leicester defender Wesley Fofana is reportedly on the summer radar for Manchester United. Transfer Talk has the latest.
Juventus forward Cristiano Ronaldo and Man United midfielder Paul Pogba will both have cameras turned on their private lives in upcoming shows.
Ole Gunnar Solskjaer has said he believes Manchester United are close to matching the level of this season's Champions League elite.
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