Global players' union FIFPRO has said FIFA's study into making the World Cup a biennial event is "inadequate" and does not address concerns over the need for a reduced workload for players.
FIFPRO said any expansion of the calendar must include proper safeguards for players' health and that the reforms should focus on the development of both men's and women's football equally.
"Proposals isolating further expansions such as a biennial World Cup -- as well as other competition reforms under discussion -- are inadequate in the absence of solutions for existing problems," FIFPRO said in a statement on Tuesday.
"Without the agreement of the players, who bring all competitions to life on the pitch, no such reforms will have the required legitimacy. The current debate once again follows a flawed process and approach."
FIFPRO general secretary Jonas Baer-Hoffmann said any plans to expand competitions must integrate the views of players.
"The lack of genuine dialogue and trust between institutions in football blocks the game's ability to build more resilience after a painful pandemic, but rather we keep reverting to the same old habits of conflict," he added.
"It is also once more frustrating that the specific and fundamental impact on the women's game is debated as a side-product rather than its own legitimate process with unique needs and opportunities."
FIFA's proposal to hold the men's and women's World Cups every two years instead of four has received mixed responses from its confederations.
European football governing body UEFA rejected the idea, with its president Aleksander Ceferin warning that European nations could boycott the World Cup if it became a biennial event, while South America's CONMEBOL said it was "highly unviable."
The Asian Football Confederation welcomed the feasibility study while the Confederation of North, Central America and Caribbean Association Football said it recognised the merits of creating a new calendar.
Confederation of African Football president Patrice Motsepe said the discussions should continue "in an open minded manner."