The European Club Association has rejected Gianni Infantino's proposal of an expanded World Cup from 32 to 48 teams.
The FIFA president has been outspoken in his desire to see the tournament grow from its current format, to 16 groups consisting of three countries each
But ECA chairman Karl-Heinz Rummenigge says it would only mean more games for players who are already over-extended as a result of long domestic seasons.
A decision by FIFA is expected in January, and the ECA have explained their stance in an official statement: "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 initiative and has today sent a letter to Gianni Infantino calling on FIFA not to increase the number of participants at future World Cups."
Rummenigge added: "We have to focus on the sport again. Politics and commerce should not be the exclusive priority in football.
"In the interest of the fans and the players, we urge FIFA not to increase the number of world cup participants."
FIFA will, however, be considering four possible formats for the World Cup when they meet in January.
Number one is to retain the World Cup as it stands, with 32 teams including eight groups of four ahead of knockout rounds.
There is a consideration for a 40-team tournament, with eight groups of five teams or 10 groups of four teams.
There is an option of 48 teams, with the competition starting with knockout matches, the winners advancing to join the other involved teams.
Infantino's preferred option is that of 48 teams divided into 16 groups of three.