FIFA's plans for the future of football would have a "direct and destructive impact" on the club game, the European Club Association has warned.
World football's governing body has been accused by the ECA of trying to "railroad" through its controversial plans - which include playing a World Cup every two years - without proper consultation.
Its powerful statement also reminded FIFA of its "legally-binding obligations" to ensure clubs are consulted over the structure of the international match calendar (IMC).
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"ECA has followed with grave concern and alarm FIFA's launch of active PR campaigns and much pretence, apparently seeking to railroad through reforms to the IMC, particularly the introduction of a biennial World Cup," the statement from the ECA, which represents 247 clubs across the continent, read.
The ECA's statement means Europe's clubs and leagues are united in their opposition to the plans, while earlier this week, UEFA also criticised FIFA over a lack of consultation and urged it to call a halt to "promotional campaigns" in support of the plans.
Via FIFA channels, former players and coaches such as Brazil's 2002 World Cup-winning striker Ronaldo, Peter Schmeichel, Tim Cahill and Avram Grant have extolled the virtues of shortening the gap between World Cups and reducing the number of in-season international breaks.
The ECA agrees with FIFA that the IMC is in need of reform. It said in its statement that a "modernised and simpler" IMC needs to be founded on fewer release windows, better player protection and health, and a balanced approach to club and international football.
The statement concluded: "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."
It will not be lost on FIFA that UEFA and the ECA's expressed concerns over player welfare come at a time when those bodies are supporting plans for an expanded Champions League from 2024-25 which would involve an extra 100 matches per season.
FIFA is in the process of engaging with all the stakeholders on the future of football in the men's and women's game, including UEFA and all the European Member Associations.
FIFA is holding an online summit next Thursday to consult with all 211 member associations.
That is in addition to separate invitations sent by FIFA on September 3 and 4 to stakeholders including all the continental confederations like UEFA, the ECA, the World Leagues Forum and world players' union FIFPRO.
The calendar reform proposals were first floated by the Saudi Arabian football association, with the FIFA Congress in May voting overwhelmingly in favour of conducting a feasibility study.
FIFA president Gianni Infantino has said he would like member associations to vote on the proposals by the end of the year.