The organisation that represents Europe's 32 leagues has reacted angrily at the prospect of a third European club competition from the 2021/22 season.
The Champions League would continue unaltered with 32 teams.
Agnelli (42), who is also president of Italian champions Juventus and sits on the UEFA Executive Committee, added that the introduction of the new competition would bring the total number of clubs competing internationally to 96 as of the 2012-22 season.
Formerly known as European Professional Football Leagues, the organisation's members include the English Football League, Premier League and Scottish Professional Football League, as well as leagues from 23 other countries.
Agnelli also hinted at the ECA General Assembly that further changes to the global calendar are being considered, despite the launch of the Nations League.
UEFA has made little secret of the fact it has been looking at ways to raise the Europa League's profile while also giving more clubs a shot at European competition, and therefore its ever-growing financial benefits, too.
News of the planned competition was first revealed last month.
When Lazio beat Mallorca 2-1 at Villa Park in Birmingham, England, in May 1999, the Roman side became the 39th and last winners of the European Cup Winners' Cup before the competition was abolished.
But concerns about diluting the Europa League's quality made the idea of a third competition more attractive, particularly if it was reserved for teams from smaller leagues.
Back then, UEFA president Aleksander Ceferin said proposals were "just discussions".