"She has been stripped of her medal," the IOC official told Reuters after an IOC Executive Board meeting on Friday.
Blonska now faces the prospect of a lifetime ban after what has officially become her second doping offence.
She tested positive after her competition on Aug 17.
All top-five finishers as well as several other random athletes undergo drugs tests during the Games.
Blonska, who tested positive to methyltestosterone, had returned to competition in 2005 after serving a two-year doping ban.
It is now up to the International Association of Athletics Federations to impose any further sanctions.
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