UEFA has opened disciplinary proceedings against Real Madrid, Barcelona and Juventus as they seek to punish the European Super League rebels.
The trio could be expelled from the Champions League for two years as a consequence.
The two LaLiga and one Serie A club are the only three of the original 12 who signed up for the breakaway competition not to have formally withdrawn.
A UEFA statement read: 'In accordance with Article 31(4) of the UEFA Disciplinary Regulations, UEFA Ethics and Disciplinary Inspectors have today been appointed to conduct a disciplinary investigation regarding a potential violation of UEFA's legal framework by Real Madrid CF, FC Barcelona and Juventus FC in connection with the so-called 'Super League' project.
'Further information regarding this matter will be made available in due course.'
The two-year ban from Europe's leading club competition is the harshest sanction at UEFA's disposal.