Thirty one athletes from 12 nations and six sports could be banned from this year's Rio de Janeiro Olympics after 454 doping samples were re-tested from the 2008 Beijing Olympics, the International Olympic Committee says.
The IOC also said on Tuesday it would start re-testing Sochi 2014 winter Games samples after allegations of tarnished samples surfaced last week.
In an effort to crack down on cheats during the Olympics, the IOC said those found to have tested positive for banned substances would not be competing in Rio.
"The aim is to stop any drugs cheats coming to the Olympic Games in Rio de Janeiro," it said.
An IOC official told Reuters no names would be made public at this stage until athletes in 12 countries had been informed and a second sample, or B-sample, tested as well.
The re-tests, a regular procedure by the IOC as it looks to re-test using newer methods or looking for new substances, were carried out in conjunction with the World Anti-Doping Agency (WADA) and international federations.
They focused on athletes who could potentially take part in the Rio Games, the first to be held on the South American continent.
Twelve national Olympic associations will be informed in the coming days, the IOC said. Another 250 samples will be re-tested from the London 2012 Olympics.
The IOC said it had called on the WADA to launch a "fully fledged investigation" into allegations that testing during the Sochi 2014 winter Olympics by the on-site accredited laboratory had been subverted.
The former head of Russia's anti-doping agency Grigory Rodchenkov claimed last week that the Sochi lab had tampered with samples.
"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," said IOC President Thomas Bach.
"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."
"By stopping so many doped athletes from participating in Rio we are showing once more our determination to protect the integrity of the Olympic competitions, including the Rio anti-doping laboratory, so that the Olympic magic can unfold in Rio de Janeiro."
