UEFA has opened proceedings against Barcelona, Real Madrid and Juventus for their role in the plans for a European Super League.
The three clubs are the only ones from the original 12 involved in the breakaway plans who have not formally withdrawn.
The nine clubs who pulled out of the European Super League - including Manchester United, Manchester City, Liverpool, Chelsea, Arsenal and Tottenham - signed a Club Commitment Declaration with UEFA earlier this month.
In a statement, the European governing body said: "Following an investigation conducted by UEFA Ethics and Disciplinary Inspectors in connection with the so-called 'Super League' project, disciplinary proceedings have been opened against Real Madrid CF, FC Barcelona and Juventus FC for a potential violation of UEFA's legal framework."
UEFA had previously opened a disciplinary investigation with regard to a potential violation of the governing body's legal framework by the three clubs concerned.
The nine other clubs committed to the existing national and international competitions, with the threat of heavy fines and expulsion were they to renew their interest in the breakaway project.
But Real Madrid, Barcelona and Juventus not only failed to follow their lead, but issued a joint statement standing by the prospective competition and its legal standing.
UEFA's move raises the prospect of the three clubs being sanctioned, with some reports suggesting they face expulsion from UEFA competitions such as the Champions League for a period of two years.