HOOFDDORP, Netherlands The players' union may ask FIFA to reconsider its decision to stage 2014 World Cup matches in early afternoon in tropical venues in Brazil.
"It's an issue for us and a serious one," FIFPro secretary general Theo van Seggelen told Reuters in an interview.
"We will discuss it and depending on the outcome we will go back to FIFA," he said before adding there had not been sufficient consultation. "It's our duty to do that to protect the game and protect the players.
"It would be unrealistic to say we would not play, that's going too far," said Van Seggelen whose organization represents 60,000 professionals worldwide.
"But we would like to know all the details and why it seems not to be a problem (to FIFA) and why we think it is a problem."
The Dutchman believes leading officials in the game must start listening to the players more.
"I know that it is sometimes difficult for football organizers but the time has passed when the players accept everything, that is finished," Van Seggelen explained.
"I am 100 percent convinced the role of FIFPro, which is already important, will be even more important in the near future. The players have been denied (a voice) for too long by the governing bodies."
Several World Cup matches will be played in the early afternoon in tropical venues such as Natal, Recife and Salvador, as well as in the dry and dusty capital Brasilia in a schedule finalized by FIFA in September.
Soccer's ruling body said it had consulted widely, including with medical specialists, but Van Seggelen explained FIFPro was conducting its own research.
"I want to know how bad it is," he added. "We realize World Cups are a real business but of course it cannot be that business is more important than the health of the players.
"I must say we have a very good relationship with FIFA when it comes to the health of the players.
"FIFA said, and I believe that, they have done some research and really understand our concerns but we think our concerns are a little bit bigger than the concerns of FIFA so we will discuss this at our next board meeting and see what we can do.
"It is not only a matter of the health of the players but is also to do with the quality of the game. The World Cup has to be played in the most ideal circumstances."
Van Seggelen acknowledged it could be difficult to make changes.
"I'm realistic, they have probably already signed the contracts," he said. "But difficult does not mean that it is impossible and we must at least do our job."
Previous World Cups have been staged in extremely hot conditions, notably the 1994 tournament in the United States and the 1970 and 1986 editions in Mexico where altitude was also an issue.
"At least we have taken a step forward," said Van Seggelen. "I think what happened in 1994 would not happen nowadays.
"The advantage we have now is there is a dialogue but it makes no sense to have a dialogue with the players if you have already made the decision before."
(Editing by Tony Jimenez)