Global players' union FIFPro has raised concerns about the planned early afternoon kick-off times at the 2014 World Cup in Brazil, insisting that temperatures in some tropical locations will make playing conditions unbearable.
FIFA has been questioned about the subject before, with the world governing body denying accusations that commercial benefits were placed ahead of player welfare when kick-off times were decided.
And FIFPro secretary general Theo van Seggelen has now returned to the issue, insisting his organisation will continue to fight to protect players, who could face temperatures of around 90 degrees in matches scheduled to start at 1300 local time (1700 BST) in host cities Natal, Recife and Salvador.
"It's an issue for us and a serious one. We will discuss it and depending on the outcome we will go back to FIFA," Van Seggelen told Reuters. "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. 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.
"We realise 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."
Twelve cities will host matches at the 2014 World Cup - Rio de Janeiro, Cuiaba, Manaus, Natal, Curitiba, Recife, Salvador, Porto Alegre, Belo Horizonte, Brasilia Sao Paulo, Fortaleza - with kick-off times at 1300, 1600 and 1900 local time, with the exception of one group match at the Arena Amazonia in Manaus, which takes place at 2200 (0200 GMT).