Only four clubs were still officially involved — Real Madrid, Barcelona, Juventus and AC Milan. The moves by Atlético and Inter came a day after the six Premier League clubs involved in the project gave up on the controversial breakaway competition.
English clubs Arsenal, Chelsea, Liverpool, Manchester United, Manchester City and Tottenham deserted the plans for the largely-closed competition amid an escalating backlash from their supporters and warnings from the government that legislation could be introduced to thwart them.
The Super League project was overseen by Real Madrid president Florentino Pérez, who promoted it as a way to "save soccer" and the clubs struggling financially amid the coronavirus pandemic.
UEFA threatened to ban players from the participating teams from playing in this year's European Championship and next year's World Cup. But a Madrid court later issued a preliminary ruling stopping UEFA, FIFA and its members from acting against the creation of the new league.
Atlético said the decision was made after its board of directors met on Wednesday.
The Spanish club said it "decided to formally communicate the Super League and the rest of the founding clubs its decision not to formalize its participation in the project."
Atlético said the "circumstances" that allowed it to join the new league on Monday "no longer existed today."
"For the club, harmony is essential for everyone involved in the (Atlético) family, especially our fans," it said. "The first team squad and its coach showed satisfaction with the club's decision, understanding that sporting merits must prevail over any other criteria."
Inter said the club was committed to delivering the best soccer experience for fans because "innovation and inclusion have been part of our DNA since our foundation."
"Our commitment with all stakeholders to improve the football industry will never change," the Italian club said. "Inter believe that football, like any sector of activity, must have an interest in constantly improving its competitions in order to continue to excite fans of all ages all over the world, within a framework of financial sustainability."
