IOC says it has opened disciplinary proceedings against the unidentified athletes from 12 countries
The International Olympic Committee (IOC) says 31 athletes who took part in the 2008 Beijing Games have tested positive for doping.
The IOC says it has opened disciplinary proceedings against the unidentified athletes from 12 countries, after a reanalysis of their samples.
"(The tests) were focused on athletes who could potentially start at the Olympic Games Rio 2016 and were conducted using the very latest scientific analysis methods," the organisation said in a statement.
"As a result up to 31 athletes from six sports could be banned from competing at the Olympic Games in Rio."
The IOC says it is determined to take tough action to fight drug cheats, adding that those who failed the tests will be informed in "the coming days".
"The fight to protect the clean athletes does not stop there, with 250 more results from retesting of samples from the Olympic Games London 2012 to come shortly," it said.
"The aim is to stop any drugs cheats coming to the Olympic Games in Rio de Janeiro."
Depending on the outcome of disciplinary proceedings, some athletes could be stripped of their medals and others given the awards.
The IOC says it cannot provide any further information on which athletes have failed the tests or which countries they come from due to legal reasons.
The doping results are the latest scandal to hit international sport, and follow new allegations involving athletes at the Sochi Winter Olympics.
A former Russian official, who now lives in the United States, claimed that up to 15 athletes were part of a state-run doping program.
The IOC says it has asked the World Anti-Doping Agency (WADA) to carry out a "fully-fledged investigation" into the allegations surrounding the Sochi Games.
It has formally asked the Russian Olympic Committee to fully cooperate with the inquiry.
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."