Super League trio Real Madrid, Barcelona and Juventus insisted on Wednesday they "remain committed to modernising football" despite UEFA's threats of disciplinary action.
UEFA on Tuesday opened formal disciplinary proceedings against the three clubs which are still refusing to give up the aborted Super League project.
In a joint statement, Italian's Juventus and the two Spanish sides said they "remain committed to modernising football through an open dialogue with UEFA".
"FC Barcelona, Juventus FC and Real Madrid CF wish to express their absolute rejection of the insistent coercion that UEFA has been maintaining towards three of the most relevant institutions in the history of football," the statement read.
"This alarming attitude constitutes a flagrant breach of the decision of the courts of justice, which have already made a clear statement warning UEFA to refrain from taking any action that could penalise the founding clubs of the Super League while the legal proceedings are ongoing.
"Therefore, the opening of disciplinary proceedings by UEFA is incomprehensible and is a direct attack against the rule of law that we, the citizens of the European Union, have democratically built up, while constituting a lack of respect toward the authority of the courts of justice themselves."
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.
The withdrawal of the six English participants after 48 hours led to the project being shelved.
The two Milan clubs followed and the nine agreed in early May to pay a combined 15 million euros 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.
"From the beginning, the Super League has been promoted with the aim of improving the situation of European football, through permanent dialogue with UEFA and with the objective to increase the interest in the sport and to offer fans the best possible show," Wednesday's statement read.
"This objective has to be achieved in a framework of sustainability and solidarity, especially in a precarious economic situation such as the one many clubs in Europe are currently experiencing.
"Instead of exploring ways of modernising football through open dialogue, UEFA expects us to withdraw the ongoing court proceedings that question their monopoly over European football.
"Barcelona, Juventus and Real Madrid, all of them more than a century old, will not accept any form of coercion or intolerable pressure, while they remain strong in their willingness to debate, respectfully and through dialogue, the urgent solutions that football currently needs.
"Either we reform football or we will have to watch its inevitable downfall."