The International Olympic committee says 31 athletes from six sports could be banned from this year's Rio Olympics after failing dope tests when 454 samples were re-examined from the 2008 Beijing Games.
The IOC also said it would start re-testing Sochi 2014 winter Games samples after allegations of tarnished samples were made last week by Russia's former top anti-doping scientist.
Some 250 samples from the London Games will also be re-examined.
In an effort to crack down on cheats during the Olympics, the IOC said this was targeted re-testing on athletes likely to be at the Rio Games starting on August 5th, and those found to have tested positive would not compete.
An IOC official told Reuters no names would be made public at this stage until athletes had been informed and a second sample, or B-sample, tested as well.
"The aim is to stop any drugs cheats coming to the Olympic Games in Rio de Janeiro," the IOC said. "The (IOC) Executive Board agreed unanimously to initiate proceedings immediately, with the 12 National Olympic Committees concerned informed in the coming days."
The re-tests, a regular procedure by the IOC as it looks to use newer methods or look for new substances, were carried out in conjunction with the World Anti-Doping Agency (WADA) and international federations.
The IOC said it had also called on 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 said last week that the Sochi lab had tampered with samples.
Russia is at the heart of the biggest drugs scandal in years, with the country's track and field athletes currently suspended and the Rio Games hopes in doubt, and their drugs testing lab and anti-doping agency undergoing complete overhaul.
"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."
Apart from tarnishing any competition, doping has also damaged the Olympics' reputation, with the IOC regularly stripping athletes of their medals, sometimes years after they competed, due to positive drugs tests.
"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," Bach said.
The Rio Games run from August 5th to 21st.