Madrid -European clubs demanded on Tuesday that UEFA "properly include" them in any plans to change the Champions League, which could include guaranteeing qualification for elite teams.
La Liga president Javier Tebas, who was among more than 300 representatives that attended a meeting in Madrid, also said "legal possibilities have been studied" in case UEFA press ahead with unwanted reforms. Tebas said he was confident "any court case would be successful".
The event brought together 244 clubs from 38 countries, with the aim to establish common ground ahead of a meeting on Wednesday between the European Leagues and UEFA's executive committee in Nyon, Switzerland.
Change to European club competitions has become a controversial issue after Andrea Agnelli, the president of the European Club Association (ECA), which represents Europe's top teams, outlined proposals for a new 'super-league' that could ensure qualification for certain clubs and feature European fixtures at weekends.
But, following the four-hour meeting on Tuesday, Tebas said in a press conference: "It is not possible to make reforms without the agreement of the leagues. I don't know how is it possible to change anything without the agreement of the national competitions."
European Leagues chairman Lars-Christer Olsson added: "The European Leagues and all our member clubs have said we have to be properly included in the decision making about the future of European competitions.
"The process has to change from informing us to properly negotiating with us."
Agnelli, who is also the president of Juventus, had told ECA clubs to boycott the meeting in Madrid but attended himself, along with ECA vice-chairman and Ajax chief executive, Edwin van der Sar.
Van der Sar sought to ease concerns about European games being played at weekends. "That is not part of the plans," he said.
"What we are fighting for and working towards is more access to European football for more countries and more clubs.
"It's important to play more interesting and meaningful games, and sometimes that doesnt happen in the (national) league."
On Agnelli's contribution to the meeting, Olsson said: "He explained these were only ideas, that they were open to discussion. He did not speak in an aggressive tone."
Tebas claimed legal action is a possible option to block the plans. "Legally we have studied the possibilities about a violation of competition rules and we continue to work on this," Tebas said. "The information we have is that any court case would be successful."
Olsson was clear about the case the European Leagues would present to UEFA on Wednesday.
"I think I can say with a lot of confidence that the vast majority of clubs at the meeting today say they are not in favour of the changes outlined by the ECA president," Olsson said.
"They don't want to see promotion and relegation or a pyramid system or a closed league in Europe and definitely not a closed league within the umbrella of UEFA," Olsson said.
"You should qualify for UEFA competitions via the domestic leagues. Otherwise it is impossible to keep the interest of the fans."
Tebas also warned about the financial impact on Spanish football if clubs like Real Madrid and Barcelona split to join a separate league.
"If the project means a (reduced) national league of 18 clubs that is almost closed off from European competitions, we did a report with KPMG and the value of the clubs in Spanish football would decrease by 45 per cent," Tebas said.
"The revenue that it would cost us would be around €800 million per season."