Real Madrid, Barcelona and Juventus on Wednesday said they won't bow down to UEFA's threats and will continue to fight for reform in football despite the possibility of being thrown out of the Champions League next season.

UEFA on Tuesday opened disciplinary proceedings against the three clubs over their attempts to launch a breakaway Super League but the teams said they rejected the "insistent coercion" the body has maintained while the matter is still in court.

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The clubs are the only three of the original 12 to have not distanced themselves from the project following an outpouring of criticism.

Sources told ESPN earlier this month that the governing body could punish the clubs by banning them from European competition for up to two years. However, Barca, Madrid and Juve warned UEFA against taking any action while they wrangle over the Super League remains with the courts.

A Madrid court has asked the European Court of Justice to establish if FIFA and UEFA are breaching EU competition law by preventing clubs from creating a breakaway European Super League.

"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," the statement read.

"Therefore, the opening of disciplinary proceedings by UEFA is incomprehensible and is a direct attack against the rule of law... while constituting a lack of respect toward the authority of the courts of justice themselves.

"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."

The Super League was launched with 12 clubs as founding members, but nine of them -- Manchester United, Liverpool, Manchester City, Chelsea, Tottenham Hotspur, Arsenal, AC Milan, Inter Milan and Atletico Madrid -- have since backed out and reached a deal with UEFA.

The six English clubs involved in the project, in addition to Atletico Madrid, Inter Milan and AC Milan, all agreed to pay a combined fine of €15 million for their involvement in the breakaway.

They will also have a five percent reduction on earnings from UEFA competitions for one season and will have to pay a fine of up to €100m if they seek to play in an unauthorised competition again in the future.

However, Barca, Madrid and Juve remain committed to the project and warned of football's "inevitable downfall" if UEFA don't enter dialogue over reforms.

"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," the statement said.

"Either we reform football or we will have to watch its inevitable downfall."

UEFA president Aleksander Ceferin warned the clubs last month that "if they say we are a Super League, then they don't play Champions League, of course."

Information from Reuters was used in this report.