Uefa has opened formal disciplinary proceedings against Real Madrid, Barcelona and Juventus, the three rebel clubs refusing to abandon the Super League project.
The Spanish giants and Italian superpower were instrumental in the formation of the proposed breakaway league announced last month.
But while nine others - Arsenal, Chelsea, Liverpool, Manchester City, Manchester United and Tottenham Hotspur of England, Spain's Atletico Madrid and AC and Inter Milan of Italy - all reneged on joining the Super league following fierece backlash from supporters - Real, Barca and Juve insist the project will still go ahead.
"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 said.
The other nine promoters of the Super League escaped prosecution in exchange for light financial penalties, seen by many as too lenient.
Uefa's preference has always been to negotiate the surrender of the rebels rather than crack down or expel them from their own competitions - the most extreme of the sanctions open to it. Between them, Real, Barca and Juve have won more than 40 Uefa competitions, including 20 European Cups.
But having failed to obtain a surrender from the three holdouts, European football's governing body has opted for disciplinary measures without specifying what offences the clubs have committed.
Uefa's statutes prohibit any "grouping or alliance" between clubs without its authorisation.
By announcing their own private Super League on the night of April 18-19, the 12 clubs turned European football on its head and threatened the very existence of Uefa.
Earlier this month, the nine agreed to pay a combined €15 million ($18.3m) and to forgo five per cent of their European revenue for one season.
Real, Barcelona and Juventus meanwhile retaliated through a Commercial Court in Madrid which referred the matter to the European Court of Justice, asking if Uefa was abusing its "dominant position" by seeking to block a competing tournament.