Global players' union Fifpro believes Fifa considers the demands of TV companies of "greater importance" than the health and safety of the players.
Fifa said this week that it will not change the 2014 World Cup kick-off times despite concerns about the heat.
Games kicking off at 13:00 in the north west of Brazil will likely take place in very hot and humid conditions.
In a statement, Fifpro said it wants "effective measures to guarantee [players'] health and safety".
Manaus, where England will meet Italy in their opening World Cup match, is likely to cause the biggest problems in terms of humidity, where temperatures can reach well in excess of 30C.
Four games will be played in the city's specially constructed Arena da Amazonia stadium during the tournament.
The southern cities of Rio de Janeiro, Sao Paulo, Curitiba and Belo Horizonte are predicted to have more comfortable temperatures (mid-20s) for much of the tournament.
The statement added: "Fifpro - the voice of all players worldwide - urges Fifa once again to consider the health and safety of the players during the 2014 World Cup as the number one priority.
"Fifa has stated to stick to the original time schedule, thus giving the players the impression that the world's governing football body considers the demands of TV companies of greater importance than the health and safety of the players."
Fifa says additional cooling breaks will be granted after the 30th minute of the first and second halves of the game if temperatures exceed the Web Bulb Globe Temperature (WBGT) - an index used for the assessment of heat stress - of 32C.
But Fifpro added: "We welcome the fact that Fifa admits that measures need to be put in place to guarantee the players' health.
"But the World Footballers' Association is of the opinion that the aforementioned additional cooling breaks are insufficient.
"Fifpro advises FIFA to allow additional cooling breaks at a lower WBGT, namely from 27C.
"Guidelines from the highly-respected American College of Sports and Medicine, indicate that all continuous activity and competition activities must be cancelled at the WBGT level that exceeds 27C."
"Players will be subjected to unreasonable health risks if no additional measures are taken by Fifa while their performances might be significantly impaired.
"In Brazil, the national football authorities do not organise any high-level matches at 13:00 hours in the north east, mid-west and south east regions, to protect the players' health."