The European Club Association (ECA) said on Sunday it "strongly opposes" plans for a breakaway Super League competition featuring 12 of the continent's biggest clubs.
A dozen sides from England, Spain and Italy have announced plans for the competition, which has been heavily criticised by UEFA and the three countries' football authorities.
"ECA as the body representing 246 leading clubs across Europe reiterates its stated commitment to working on developing the UEFA Club Competitions (UCCs) model with UEFA for the cycle beginning 2024 and that a 'closed super league model' would be strongly opposed by ECA," the body said in a statement.
The ECA, often seen as the voice of the most powerful European clubs, took this strong position just before 12 of the biggest European clubs announced the launch of the breakaway Super League.
A position all the more surprising given that among these clubs is Juventus, chaired by Italian Andrea Agnelli, who was also the president of the ECA.
However, the Italian club confirmed on Sunday it has left the ECA and that Agnelli has resigned from the presidency of the association.
Agnelli has also resigned as a member of the UEFA executive committee, a Juventus spokesperson told AFP, confirming the Turin club's "membership in the new Super League".
Earlier on Sunday, European football's governing body UEFA said clubs could be banned from domestic competitions and the Champions League if they take part.
"The ECA Executive Board will be convening over the coming days to take appropriate decisions in light of any further developments," the ECA added.
World football governing body FIFA expressed its "disapproval" at the announcement.
FIFA stressed in a statement it was in favour of "solidarity" in the sport and said: "Against this background, FIFA can only express its disapproval to a 'closed European breakaway league' outside of the international football structures and not respecting the aforementioned principles."