A fire in the basement of the Arena Pantanal at Cuiaba in Mato Grosso state could scupper the 2014 World Cup venue's chances of being ready by FIFA's December deadline.
Three fire engines were called to put out the blaze that started when polystyrene insulation panels caught alight, officials said. No one was injured.
Although World Cup organisers said the accident would not halt construction work, it came 24 hours after the state governor warned the stadium might not be 100 percent complete in time for the deadline set by soccer's governing body.
The Arena Pantanal, situated 1,300km north-west of Sao Paulo, is one of six unfinished venues due to be used in the finals. Four tournament games will be played there.