Ukraine's Lyudmyla Blonska, the heptathlon silver medallist, has failed a drugs test, the IOC said on Wednesday.
The 30-year-old's A sample tested positive for a banned substance, but the results of the B sample are still awaited.
A test conducted at 2am on Sunday has shown traces of the anabolic steroid Methytestosterone. Blonska is still waiting for the results of the B Sample.
Lyudmyla Blonska had already served a previous ban for testing positive in 2003 for stanozolol.
Blonska now faces a life ban for the latest offence.
Should Blonska be stripped of her medal, the silver would go to American Hyleas Fountain while Tatiana Chernova of Russia would get the bronze.