ECA president Andrea Agnelli - who also holds the same title at Juventus - is keen to introduce a Champions League format that virtually closes off the elite competition via a promotion and relegation system with the second-tier Europa League.
European clubs and leagues are arguing about the future shape of the Champions League.
If accepted, the new proposal could see the existing 32-team competition expand, to include either 40 or 48 teams in the group stages from 2024.
However, there is still a big push for change and Uefa are set to discuss the expansion soon.
A format with 40 or 48 teams would favour not only Europe's wealthiest clubs but also traditional giants, such as Ajax and Celtic, who do not now qualify for the Champions League group stage by winning their domestic leagues.
Fifteen Serie A clubs have come out against the European Club Association's controversial plans to reform the Champions League, which are being backed by Italian giants Juventus, according to reports in Italy on Tuesday. They were the latest of Europe's major leagues to reject the ECA reform plans.
But Agnelli's proposition drew criticism for favouring Europe's biggest clubs and creating a closed off "Super League". Subsequently, all 20 of England's Premier League teams opposed it.
A third-tier competition will start in 2021 with 32 teams, and it could double in size three years later to help clubs from smaller countries stay involved beyond early-season qualifying rounds.
The ECA concept circulating since March calls for eight-team groups instead of four.
"We also express our concerns about the economic and competitive impact on Serie A of this UEFA reform", they said.