The European Club Association (ECA) has written to FIFA imploring global football's governing body not to proceed with plans to expand the World Cup.
When running for election, FIFA president Gianni Infantino's manifesto included a proposal to increase the number of teams in the finals from 32 to 40.
And in October, Infantino hinted it could be expanded even further to include as many as 48 countries.
However, the ECA issued a statement on Thursday voicing its opposition against such plans, while also revealing it had written to Infantino in a bid to persuade him against the idea.
"The European Club Association (ECA) speaks out against the proposed enlargement of the FIFA World Cup," the statement read.
"FIFA President Gianni Infantino has recently made several proposals aimed at further expanding the number of participating nations at the World Cup. According to Gianni Infantino's proposals, the expansion is expected to take effect at the FIFA World Cup in 2026.
"The number of games being played throughout the year has already reached an unacceptable level, in particular for national team players.
"The ECA Executive Board therefore rejects such [an] initiative and has sent a letter to Gianni Infantino calling on FIFA not to increase the number of participants at future World Cups."
"We have to focus on the sport again. Politics and commerce should not be the exclusive priority in football," added ECA chairman Karl-Heinz Rummenigge.
"In the interest of the fans and the players, we urge FIFA not to increase the number of World Cup participants."