Rio 2016: Up to 31 athletes could be banned after Beijing retests
Up to 31 athletes from six sports could be banned from competing at the Rio Games, Olympic chiefs have said.
The announcement comes after the International Olympic Committee (IOC) retested 454 selected doping samples from the 2008 Games in Beijing.
The IOC said the retests were conducted using the very latest scientific analysis methods.
It also revealed it is awaiting the results of 250 retests from the 2012 Olympics in London.
"All these measures are a powerful strike against the cheats we do not allow to win," IOC president Thomas Bach said.
"They show once again that dopers have no place to hide. We keep samples for 10 years so that the cheats know that they can never rest.
"By stopping so many doped athletes from participating in Rio, we are showing once more our determination to protect the integrity of the Olympic competition."
More than 4,500 tests were carried out at the Beijing Games in 2008 but just nine athletes were caught cheating.
The IOC said the retests were focused on athletes who could potentially take part in Rio.
It added 12 affected national Olympic associations would be informed in the coming days.
However, the IOC said it would not be revealing the names of athletes who had returned adverse findings until B-samples had been tested and individuals informed.
The British Olympic Association said it has not been contacted by the IOC.
The organisation also confirmed it is to start re-testing samples from the Sochi 2014 Winter Olympics.
Last week, a whistleblower alleged Russian secret service agents helped to protect drug cheats in Sochi, although the Russian authorities denied the claims.
The World Anti-Doping Agency (Wada) is now investigating those allegations and on Tuesday announced it had appointed former Interpol agent and French Gendarmerie major Mathieu Holz to lead the inquiry.
The latest measures taken by the IOC come after Russia and Kenya were found to have breached anti-doping rules in recent months.
Russia was banned from athletics competition in November after a Wada commission report recommended the sanction.
Russian sports minister Vitaly Mutko said his country was "very sorry" and "ashamed" of cheating athletes who were not caught by its anti-doping systems.
But he argued not lifting the ban for the Rio Games would be "unfair and disproportionate" and that clean athletes should not be punished.