UEFA president Aleksander Ceferin has admitted his "grave concerns" over plans to stage the World Cup every two years.
In a letter to Football Supporters Europe executive director Ronan Evain, seen by the PA news agency, Ceferin described fans' concerns over FIFA's proposals as "extremely valid and important".
The FIFA Congress voted in May to conduct a feasibility study into holding the men's and women's World Cups every two years after a proposal from the Saudi Arabian Football Federation, and Arsene Wenger, the world governing body's chief of global football development, was quoted in the French press on Friday promoting the idea.
Ceferin said that UEFA and its national associations had "serious reservations and grave concerns surrounding reports of FIFA's plans".
He said there was "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 and all the football community".
He added: "As one concrete example amongst so many, it is imperative to highlight the concerns shared across the footballing world regarding the impact a biennial FIFA World Cup would have on the international match calendar and, prominently in this context, on women's football."
Ceferin was responding to a letter from Evain, also seen by PA, which emphasised Football Supporters Europe's "firm opposition" to the World Cup plans and urging UEFA to "challenge them in the interest of the global game".
Evain said a biennial World Cup would have an "adverse impact on the balance between local, domestic, continental and international competitions" and "likely undermine" tournaments like the European Championship and Copa America.
"Moreover, most fans look forward to the World Cup precisely because it is a unique event that only occurs every four years," Evain added, pointing out fans do not have "an unlimited amount of time, money or enthusiasm to expend on flights, accommodation and tickets — or TV subscriptions".
He said: "There is no doubt that football is in desperate need of reform – across the UEFA region and beyond. As things stand, the game is unequal, expensive to watch and, for the millions who do not have access to adequate grassroots facilities, difficult to play. But doubling the number of World Cups will not solve any of these problems. In fact, it will inevitably make them worse."
In his reply, Ceferin said: "UEFA fully supports your intentions, in cooperation with fans' organisations, to closely scrutinise and hold to full account any developments in this area."
He added: "UEFA stands with you and the fans on this important issue."
FIFA said on Friday that the World Cup feasibility study had "no predetermined objectives" and that fans were being consulted "on an unprecedented scale".
A spokesperson said: "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.
"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."
