UEFA's Professional Football Strategy Council (PFSC) have expressed "serious reservations" over FIFA's proposal to revamp the Club World Cup and develop a new Nations League.
The proposals were put forward by FIFA President Gianni Infantino with a consortium reportedly backing the tournaments.
FIFA are keen to introduce the events as they could generate $25 billion (£18 billion/€20.5 billion) in revenue through broadcasting rights in Asia alone.
The new 24-team Club World Cup would take place in June or July every four years and feature 12 teams from Europe.
These would be the Champions League finalists and Europa League winners from the previous four seasons, with the European contingent joined by four clubs from South America and two each from Africa, Asia and North America.
One club would also come from the host country with the last side from either South America or Oceania.
Opposition has risen to the proposals, with the PFSC claiming the "hasty timing and lack of concrete information" around the process had concerned them.
The PFSC consists of elected representatives of the four main stakeholder groups involved in European professional football.
This includes the European Club Association, European Leagues, UEFA and the players, through FIFPro Division Europe.
UEFA President Aleksander Ceferin was among the attendees at a meeting of the group.
"The PFSC unanimously expressed serious reservations about the process surrounding the FIFA Club World Cup and Global Nations League proposals and in particular the hasty timing and lack of concrete information and underlined the need for a clearly defined procedure, which respect existing structures and decision-making bodies and which involves all key stakeholders," a statement read.
"In particular, such proposals must be considered as part of a global reflection on the overall international match calendar and cannot be decided upon in isolation."
The meeting took place prior to the Europa League final in Lyon, which saw Atletico Madrid earn a 3-0 victory over French side Marseille.
FIFPro issued a separate statement following the meeting, claiming FIFA's proposals would have a "very significant impact on the future of professional football worldwide".
"It is FIFPro's obligation and interest to contribute to the sustainable growth of football, to the benefit of all member players and professional competitions that provide decent employment conditions," FIFPro said.
"FIFPro is analysing how the proposed competitions would benefit this development as well as their impact on those players participating in those matches.
"FIFPro is taking the time needed to gather and assess the views of its unions and consult with players directly about the potential impact on their working lives.
"Critical issues such as player workload and the amount of recovery time players need between matches will be factored into FIFPro's decision-making process."
La Liga champions Barcelona and current Champions League holders Real Madrid have reportedly expressed their support for the revamped Club World Cup.
The competition is currently a seven-team tournament, held in December each year.
The Nations League would also be introduced by FIFA, who would scrap the Confederations Cup, which takes place a year prior to the World Cup.