ECA, led by Juventus chairman Andrea Agnelli, is closely involved in proposals to reshape Europe's main club competition from 2024.
The proposals were presented by European football's governing body UEFA in May although details had leaked earlier.
The reforms include the introduction of weekend fixtures, four groups of eight, and a tiered system with relegation and promotion that would see the top six teams in each group automatically qualify for the following year's competition.
ECA, which boasts of "more than 230 members", faced an outcry from many of those clubs and from national leagues which would be hit hard by the scheme.
"A semi-closed league with more matches... threatens to enormously impoverish the Spanish league," said Javier Tebas of La Liga.
England's Premier League issued a statement saying: "The domestic game should continue to be the priority for professional clubs."
Opponents say the plan is designed to guarantee the income of a handful of big clubs.
"This reform would especially harm medium and small clubs," said Wanja Greuel, the president of Young Boys of Berne, the reigning Swiss champions. "It eliminates the access to the top flight of European competitions through domestic leagues. Fans will gradually lose interest in domestic leagues and those leagues would be further economically damaged."
Last month UEFA announced it was cancelling a meeting with the ECA and the European Leagues, a wider body representing more than 900 European clubs, scheduled for September 11.
UEFA President Aleksander Ceferin said he wanted to "collect feedback" from member federations.
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