Uzbek gymnast Luiza Galiulina has been provisionally suspended after testing positive for the banned diuretic furosemide, the International Olympic Committee (IOC) announced on Sunday.
Galiulina underwent a drugs test on July 25, four days before the start of the women's artistic gymnastics competition.
"Once the results of the B sample have been received, a definitive decision will be announced by the disciplinary commission," said the IOC.
Like some other diuretics, furosemide is listed on the World Anti-Doping Agency's banned drugs list as it can be used as a masking agent to disguise the use of other drugs.
It is the second case of doping at the London Olympics, after Albanian weightlifter Hysen Pulaku tested positive for a banned steroid.
More than a dozen other athletes have already been expelled for pre-competition doping offences, but Pulaku, 19, was the first to fail a test since arriving for the Games.
Among the pre-Games doping casualties were Morocco's Mariem Alaoui Selsouli, one of the favourites for the women's athletics 1500m gold medal, who failed a test for a banned diuretic.
World anti-doping chiefs have unveiled a new test for human growth hormone, with a detection window of weeks rather than hours, which they hope will snare drug cheats at the Games.
A total of 15 athletes were caught doping at the 2008 Beijing Olympics, the most at a Summer Games since the IOC began outlawing drugs at the 1968 edition in Mexico City.
Uzbek athlete Luiza Galiulina arrives at Heathrow airport in London on July 24, three days before the start of the London Olympic Games. Galiulina has been provisionally suspended after testing positive for the banned diuretic furosemide, the International Olympic Committee (IOC) announced on Sunday.