Greece 200m sprinter Anastasios Gousis has tested positive for drugs and will not compete in Beijing after being sent home from his training camp in Japan.
The tests were conducted before Gousis left for his team's pre-Olympics base.
Gousis, 29, is the first athlete to test positive since the International Olympic committee took over drugs controls for the Games on 27 July.
"Yes, there has been a case of a doping violation found by the Greek doping agency," said the IOC's Giselle Davies.
"It was within the Olympic period. But it was not an IOC test. This is a case which is under the auspices of the Greek authorities.
"The athlete will not be coming to Beijing."
Gousis could also be banned from the London 2012 Olympics under a new IOC rule that states all athletes with a suspension of more than six months are to be banned from the next Olympics as well.
A first-time doping offence usually carries a two-year suspension.
Earlier on Saturday, Greek athletics chairman Vassilis Sevastis confirmed: "We have been informed that there is a positive result involving one of our athletes."
The drug is believed to be the banned steroid methyltrienolone.
Both of the samples taken from the 29-year-old by the Greek anti-doping council (Eskan) must test positive for proof of doping to be established.
The news follows a spate of withdrawals from Greek athletes who have tested positive for banned substances.
Eleven weightlifters, a boxer and top swimmer Yiannis Drymonakos have all been caught in the past three months following tighter testing controls imposed by the Hellenic Olympic Committee (HOC).
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