Fifa's plans for a biennial World Cup would lead to a "direct and destructive impact on the club game", Europe's top clubs organisation said on Friday.
The European Club Association (ECA) said Fifa's approach to the reforms of the international match calendar were in "direct and unilateral breach of certain legal obligations", as it joined European football's governing body Uefa in opposing the proposals.
The ECA, which represents 234 European clubs, did not state what obligations it was referring to, but the organisation has made a series of agreements with Uefa and global football body Fifa regarding the calendar.
"Football clubs have always been a fundamental and respected voice in shaping the future of the IMC (international match calendar). They are the cornerstone of football, being the drivers of competitions; the primary hub and home of player development and investment; and the fulcrum of fans and their local communities," said the ECA.
"This is why the Memorandum of Understanding (MoU), which governs the relationship between ECA and Fifa, places the IMC at its core. This MoU between the clubs and Fifa was agreed upon following detailed negotiations and joint approval on the IMC, as well as the processes that govern the IMC."
Fifa is conducting a feasibility study into holding the World Cup on a biennial basis, a change from the current four- yearly cycle, but has made no secret of its desire to switch to such a format.
Arsene Wenger, the former Arsenal manager who is now Fifa's head of Global Football Development, said this month he was "100 per cent convinced" about the benefits of the switch.
The ECA said it has been ready to discuss a "modernised" calendar, but there has been a lack of consultation from Fifa.
"ECA has therefore 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," it said.
On Monday, Fifa said it had "reached out to its member associations and other stakeholders (representatives of the players, clubs, leagues, confederations) marking the beginning of a new phase of consultation".
Uefa president Aleksander Ceferin warned this month of a potential European boycott of the World Cup if Fifa's plans went ahead.
While South American confederation Conmebol has also been strongly critical of Fifa's approach, the Asian Football Confederation (AFC) said it "welcomes the extensive consultation process initiated and led by Fifa".
Concacaf, which governs the game in North and Central America and the Caribbean, said it was looking at the proposals "with an open mind, and in the spirit of positive engagement" and African confederation CAF has taken a similar stance.
Any vote on changes would likely have to be passed by the full Fifa congress of 211 national associations.