A bitter battle for control of the game and its lucrative revenue began with a letter sent by the 12 clubs to UEFA on Monday in which they said they would take legal steps in unnamed courts to protect their interests as they set up the league.
Reuters has not seen the letter but multiple sources confirmed it had been sent to UEFA and was in response to the governing body's statement on Sunday that they would "consider all measures available to us, at all levels, both judicial and sporting in order to prevent this happening".
UEFA also said the clubs concerned "will be banned from playing in any other competition at domestic, European or world level, and their players could be denied the opportunity to represent their national teams."
In the past, breakaway threats have led to compromises between UEFA and the big clubs in the Champions League, Europe's elite club competition, over the format and revenue distribution.
But this is the first time the wealthiest clubs have gone beyond threats and taken the step of creating a new league which they own and announcing concrete plans to set up a rival competition.
As well as Manchester United, owned by the American Glazer family, U.S- owned Premier League clubs Liverpool and Arsenal, Abu-Dhabi backed Manchester City, Russian-owned Chelsea and Tottenham Hotspur have signed up to the plans.
Barcelona and Atletico Madrid from Spain join Real Madrid. AC Milan and Inter Milan make up the trio from Italy along with Juventus.
Qatari-owned Paris St Germain and European champions Bayern Munich have not signed up to the plans.
The Super League said they aimed to have 15 founding members and a 20-team league with five other clubs qualifying each season and would start playing as "soon as is practicable".
The founding members will have automatic places in the Super League and would not need to qualify -- nor could they be relegated.
The 15 clubs will also own the league through a private company and will share 3.5 billion euros ($4.21 billion) to spend on infrastructure and recovery from the impact of the COVID-19 pandemic.
French President Emmanuel Macron and UK Prime Minister Boris Johnson have condemned the plans as have the major European domestic leagues and football federations.
Soccer's world governing body FIFA had warned in January that any breakaway league would not be recognised by them and that players who took part in it would be banned from featuring in the World Cup.
However, FIFA took a softer tone on Sunday with a statement noting only its "disapproval" of a "closed European breakaway league" and with no mention of sanctions.
UEFA's ruling executive committee meeting in Switzerland on Monday was originally scheduled to rubber stamp its plans to reform and expand the Champions League.
Those plans were to expand from 32 to 36 teams and create more group stage games before the knockout rounds.
($1 = 0.8317 euros)
(Reporting by Simon Evans; Editing by Ken Ferris) ((email@example.com;))