In a release on its website, the Brazil federation said Marques' infraction occurred off the court.
Because Marques isn't injured, Brazil will have to play with 11 players in London since the team had already submitted its roster for the games and could only change it for injury.
Marques, who helped the Atlanta Dream reach the WNBA finals the past two seasons, had 21 points in Brazil's 99-67 loss to the U.S. on Monday.
"I heard about it for the first time today," said U.S. guard Angel McCoughtry, who played with Marques on the Dream. "I haven't talked to her and don't really know what's going on, but I do wish her the best."
Brazil will play Australia, Russia, Canada, Britain and France at the Olympics. The Brazilians played an exhibition game against Australia on Saturday and lost 78-75 without Marques. Brazil will have one more contest against China on Sunday before opening Olympic play next Saturday.
This wasn't the first run-in that Marques had with the Brazilian basketball federation. She wasn't part of the 2008 Olympic team after refusing to enter a game in overtime of a pre-Olympic qualifying tournament. Brazil finished 11th at the Beijing Games.