Thirty-one athletes have tested positive for doping in retesting of 2008 Beijing Games samples, the International Olympic Committee (IOC) said Tuesday.
Disciplinary proceedings against the unidentified athletes from 12 countries and six sports have been opened, the IOC said.
The samples had been stored at the IOC laboratory in Lausanne and retested using enhanced scientific methods. The IOC is further targeting potential cheats from the 2012 London Olympics and the 2014 Winter Games in Sochi, Russia.
It has retested 454 selected doping samples from the 2008 Games and focused on athletes who could potentially start at the 2016 Olympics in August in Rio de Janeiro.
"As a result up to 31 athletes from six sports could be banned from competing at the Olympic Games in Rio," the IOC said.
Meeting in Lausanne, Switzerland, the IOC executive board "agreed unanimously to initiate proceedings immediately," with the 12 national Olympic committees whose athletes are affected to be notified "in the coming days," the statement said.
"All those athletes infringing anti-doping rules will be banned from competing at the Olympic Games Rio 2016."
The IOC said 250 more results from retesting of samples from the 2012 London Games were "to come shortly."
There will be a wider retesting programme of medallists from Beijing and London. Samples of those athletes who could be awarded medals following the disqualification of others will likewise be retested.
"The aim is to stop any drugs cheats coming to the Olympic Games in Rio de Janeiro," the IOC said.
In a further measure, samples from the 2014 Sochi Winter Olympics will be retested using the latest scientific analysis.
The IOC said it had instructed the World Anti-Doping Agency (WADA) "to initiate a fully fledged investigation into allegations that testing at the Sochi laboratory was subverted."
Olympic samples are kept for 10 years "so that the cheats know that they can never rest," IOC president Thomas Bach said.
"All these measures are a powerful strike against the cheats we do not allow to win," he said. "They show once again that dopers have no place to hide."
The New York Times, citing two people familiar with the case, reported late Tuesday that US federal authorities are investigation allegations of state-led doping by top Russian athletes, on grounds of fraud and conspiracy. The Times said its sources were not authorized to discuss the probe publicly.
Russia was suspended from international competition in November after a damning report on doping by WADA.
The US investigation is reportedly being conducted by the office of the US attorney for New York's Eastern District, the same prosecutors who last year indicted top officials from football governing body FIFA on corruption charges.
Among the dozens of athletes, coaches and officials targeted in the US doping probe is Grigory Rodchenkov, former chief of Russia's anti-doping laboratory, according to the newspaper.
In a recently published interview, he told the Times that Moscow for years had ordered him to assist Russian athletes in evading bans on the use of performance-enhancing drugs.
Russia Sports Minister Vitaly Mutko, writing this week in Britain's Sunday Times, said doping was "a global problem" and a ban on Russia for the Games in Rio in August would punish clean athletes.
"We do not deny having a problem in Russia, and we are doing everything possible at state level to eradicate doping, including punishing athletes and coaches found to have violated anti-doping rules," Mutko wrote.