Thirty-one athletes have tested positive for doping in retesting of 2008 Beijing Olympic samples, the International Olympic Committee (IOC) said Tuesday.
Disciplinary proceedings against the unidentified athletes from 12 countries and affecting 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 also targeting potential cheats from the 2012 London Olympics and the 2014 Winter Olympics 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 Rio de Janeiro in August.
"As a result up to 31 athletes from six sports could be banned from competing at the Olympic Games in Rio," the IOC said in a statement after a meeting in Lausanne, Switzerland of its executive board.
"The Executive Board of the IOC today agreed unanimously to initiate proceedings immediately, with the 12 NOCs concerned informed 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 re-testing programme of medallists from Beijing and London. Samples of those athletes who could be awarded medals following the disqualification of others will also 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 also be retested using the latest scientific analysis.
The IOC said it had instructed World Anti-Doping Agency WADA "to initiate a fully fledged investigation into allegations that testing at the Sochi Laboratory was subverted."
IOC president Thomas Bach said: "All these measures are a powerful strike against the cheats we do not allow to win.
"They show once again that dopers have no place to hide. The re-tests from Beijing and London and the measures we are taking following the worrying allegations against the laboratory in Sochi are another major step to protect the clean athletes irrespective of any sport or any nation.
"We keep samples for 10 years so that the cheats know that they can never rest."