Enrico Carpani, a spokesman for the governing body, told The Associated Press that Baranova tested positive for testosterone July 24 in Belarus. The test was made at the request of the International Olympic Committee as part of pre-games testing.
Carpani said Baranova had already left the games.
"She admitted her guilt — there was no problem with that — and she left the games a couple of days ago," he said.
The 22-year-old Baranova was the bronze medallist in the sprint at the European championships, and the two-time under-23 European champion in both the sprint and the keirin.
Baranova, who is coached by her father, Alexei, is listed by the sport's governing body as the No. 2-ranked sprinter and keirin rider in the world.
Baranova, who was set to compete in the keirin event in London, told the Ria Novosti news agency that she thought she would not be caught by anti-doping authorities.
"At the chemist, I bought the tablets containing testosterone of my own accord," she said. "Since I knew it was prohibited, I didn't mention anything about it to my coach. I took one tablet."
Baranova said she was required to take the doping test on the day she had taken the medicine.
"I was scared and instantly regretted this course of action, but I didn't tell anyone," she said. "Right before leaving for the Olympic Games another doping sample was taken, and it was clean. I thought I got lucky and nothing would happen."