Fifa's calendar revamp would "lead to a direct and destructive impact on the club game", says the European Club Association.
World football's governing body has proposed a men's World Cup every two years as part of an overhauled international match calendar (IMC).
The ECA said the planned changes were "in direct and unilateral breach of certain legal obligations".
It also criticised Fifa for a lack of consultation about its reforms.
In an exclusive interview with BBC podcast The Sports Desk, Arsene Wenger - Fifa's head of global development - said proposals would tackle "chaos" and "congestion" in the game.
The ECA, which represents more than 230 clubs across the continent, said that that "changes are needed" to the IMC to make it "modernised and simpler" but was unhappy with the plans and the way Fifa was going about it.
In a statement, the ECA said it had "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".
European football's governing body Uefa has said the plans created four significant "dangers" for football, including a potential loss in prestige for the World Cup and concerns over player welfare.
It rejected Wenger's suggestion the new schedule would improve the competitive chances of smaller nations, and said the development of women's football, with tournaments "deprived of exclusive slots and overshadowed by the proximity of top men's events", would be impacted.
The ECA added: "Aside from the notable lack of genuine (or indeed any) consultation, and as many stakeholders have pointed out in recent days, Fifa's proposals would lead to a direct and destructive impact on the club game, both domestically and internationally.
"In addition, the proposals would put players' health and wellbeing at risk. They would dilute the value and meaning of club and country competitions.
"They would diminish and conflict with women's and youth football - being under-represented areas of the game where ECA and others are committing significant time, talent and resources - while also subordinating other sporting tournaments and interests at all levels worldwide.
"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."
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