Italian race walker Alex Schwazer will not defend his title in the 50-kilometre race on Saturday after failing a drugs test.
The Italian Athletics Federation (FIDAL) announced Schwazer has been pulled out of their Olympics squad.
"This afternoon, FIDAL has received confirmation of the positive doping test of Alex Schwazer and the subsequent decision taken by CONI (the Italian Olympic Committee) to exclude him from their Olympic team," read a statement on the FIDAL website.
Schwazer was also entered in the men's 20km walk which took place on Saturday but withdrew last month, citing a cold.
At the time the 27-year-old made it clear he intended to defend his 50km title. He won silver at the shorter distance in Beijing.
Schwazer told Italian press agency ANSA: "I made a mistake, my career is ended here.
"I did it all myself and on my own head so I assume all the responsibility for what has happened."
FIDAL president Franco Arese said: "Better that it happened before the race.
"If it had happened afterwards it would have been worse. We agree with CONI's decision to exclude him immediately from the Olympic team.
"He will be severely punished, as must be obvious, but I see from what he has said that he has had the courage to take full responsibility.
Arese said: "I can't understand what made him do it. I am sure he did not need to. One day I will ask him."
Earlier, American judoka Nick Delpopolo was expelled from the Games after a positive drugs test, and admitted he had eaten food which contained marijuana.
Delpopolo insisted he had not known the food contained the banned substance.
The 23-year-old competed in the men's 73kg division and was tested on July 30 after his loss to Mongolian Nyam-Ochir Sainjargal in the repechage stage, which put him out of medals contention.
The International Olympic Committee said Delpopolo tested positive for 11-nor-delta-9-tetrahydrocannabinol-9-carboxylic acid, which is connected to marijuana consumption.
According to the IOC ruling, Delpopolo "explained that his positive test was caused by his inadvertent consumption of food (prior to his leaving for the London Olympic Games) that he did not realise had been baked with marijuana".
Delpopolo apologised to the United States Olympic Committee and team-mates and, according to the IOC, "indicated that he was embarrassed by this mistake".
The IOC disciplinary commission that heard the case stated that Delpopolo should have his seventh-place finish rescinded, return his diploma for competing in the Games and have his accreditation withdrawn.