The group which represents elite European football clubs says FIFA's proposal to hold the World Cup every two years would be "destructive".
The European Club Association's accused the world governing body of trying to "railroad" through its plans, which are led by former Arsenal boss Arsene Wenger.
The ECA has reminded FIFA it has "legally binding obligations" to consult clubs on the future of the international match calendar (IMC).
"ECA has followed with grave concern and alarm FIFA’s launch of active PR campaigns and much pretense, apparently seeking to railroad through reforms to the IMC, particularly the introduction of a biennial World Cup," the representative body said in a statement.
"Given the centrality of the IMC to club football, and of club football to the IMC, ECA reiterates in the most unambiguous terms that any decisions relating to its future can only come about with the consent of football clubs, with player welfare at their heart – and in keeping with legally-binding obligations which ordinarily should not need restating.
"In short, the reform of the IMC must be founded on jointly-agreed outcomes, balanced in the interests of all, following honest and detailed consultation – not simply railroaded in the singular interests of FIFA on the back of a series of PR campaigns."
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