UEFA will come out strongly next week against plans by FIFA to hold a World Cup finals every two years.
UEFA president Aleksander Ceferin spelled out his opposition to the biennial finals in a response to a letter from fans group Football Supporters Europe.
Ceferin said UEFA has "serious reservations and grave concerns surrounding reports of FIFA's plans".
"Considering the major impact this reform may have on the whole organisation of football, there is widespread astonishment that FIFA appears to be launching a PR campaign to push its proposal whilst any such proposals haven't been presented to confederations, national associations, leagues, clubs, players, coaches, clubs and all the football community," the letter read.
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FIFA's chief of global football development Arsene Wenger is the driving force behind the plans which would need to be voted on by the 211 national football federations and associations which make up FIFA's congress.
Football's world governing body is carrying out a feasibility study into the proposal put forward by the Saudi Arabian Football Federation.
"In May, 166 member associations voted to carry out a feasibility study on the impact of playing the FIFA World Cup and FIFA Women`s World Cup every two years. Under the leadership of Arsene Wenger, FIFA's Chief of Global Football Development, a consultation process is ongoing and will continue in the coming months with all key stakeholders, including confederations and member associations," a FIFA spokesperson said.
"FIFA encourages everybody to share their points of view in a positive spirit of dialogue. We note with satisfaction the willingness from stakeholders to discuss the International Match Calendar and other important issues. Fans around the world are being consulted on an unprecedented scale and FIFA will publish results in due course. The aim of the consultation is to conduct a comprehensive analysis taking into account the diverse views and interests of global football.
"There are no predetermined objectives, and FIFA has an open mind in search of better solutions for the common good of the game."
Many federations are in favour of the proposal because it would increase revenues and give them more chance of playing in the finals. Opponents, including UEFA, European clubs and leagues, are likely to argue that more games would be impossible to fit into an already congested match calendar and harm the welfare of players.
Wenger would also like all qualifying games for major tournaments to be played in one block in October.
Major women's tournaments, such as the World Cup and continental events like the European Championship, are currently held in odd-numbered years. The men's World Cup and Euros are held in even-numbered years. FIFA's new vision for football would mean every year would feature a men's tournament, which would deny the clear summer focus to promote the growth of the women's game.
A World Cup would also clash with the Olympics, unlike now.