Real Madrid, Barcelona and Juventus, the three clubs still involved in the proposed European Super League, denounced on Saturday the "intolerable" pressure put on them by UEFA to drop the project.
UEFA said on Friday that the three clubs will face sanctions and European football's ruling body has started disciplinary proceedings against them. A breakaway Super League set up by 12 clubs was announced last month but fell apart after 48 hours.
The nine other clubs, including six English Premier League teams, backed out of the plan and have signed a "Club Commitment Declaration" including a series of reintegration steps.
"The founding clubs have suffered, and continue to suffer, unacceptable third-party pressures, threats, and offenses to abandon the project," Real Madrid, Barcelona and Juventus said in a joint statement.
"This is intolerable under the rule of law and Tribunals have already ruled in favour of the Super League proposal, ordering FIFA and UEFA to, either directly or through their affiliated bodies, refrain from taking any action which may hinder this initiative in any way while court proceedings are pending."
The Super League argued it would increase revenue for the top clubs in Europe and allow them to distribute more money to the rest of the game.
UEFA, other teams and fan organisations, however, said the league would only boost the power and wealth of elite clubs. Juve, Real and Barca said they were ready to reconsider the proposed approach of the breakaway project following the backlash, but said it would be "highly irresponsible" for them to abandon the project.
"We are fully aware of the diversity of reactions to the Super League initiative and, consequently, of the need to reflect on the reasons for some of them. We are ready to reconsider the proposed approach, as necessary," their statement added.
"However, we would be highly irresponsible if, being aware of the needs and systemic crisis in the football sector... we abandoned such mission to provide effective and sustainable answers to the existential questions that threaten the football industry."
The nine clubs who pulled out of the Super League have agreed to "take all steps within their power" to end their involvement in the league.
They have been handed financial punishments by UEFA and have agreed to make a combined 15 million euros goodwill contribution to benefit youth and grassroots football across Europe.
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