Uzbek gymnast Luiza Galiulina became the third athlete to be expelled from the London 2012 Olympics for doping.
Her B sample also tested positive for the banned substance furosemide, the International Olympic Committee said tonight.
Uzbekistan's only gymnast at the Games had been provisionally banned on 29 July after her urine sample had showed traces of the diuretic that can be used as a masking agent for other performance-enhancing drugs.
"The athlete's file shall be transmitted to the Federation Internationale de Gymnastique (International Gymnastics Federation), which is requested to consider any further action within its own competence," the IOC said.
Galiulina, 20, who also competed at the 2008 Games, faces a maximum two-year ban as a first-time offender.
Furosemide, often sold under the brand name Lasix, can be used to treat high blood pressure, congestive heart failure or swelling caused by excess fluid retention.
It has also been used to prevent horses from bleeding through the nose during races.
Galiulina joins Albanian weightlifter Hysen Pulaku, who tested positive for an anabolic steroid, and St Kitts and Nevis sprinter Tameka Williams, who admitted taking a banned substance.
The IOC has said it expects to carry out at least 5,000 tests - 3,800 urine and 1,200 blood - during the London Games.
A drive by anti-doping agencies across the world has yielded more than 100 cheaters ahead of the Games as the IOC seeks to avoid major doping scandals that hit previous editions of the multi-billion dollar event and overshadowed the competitions.