The IOC said its re-tests focused on athletes slated to travel to Rio de Janeiro for the summer Games in a few weeks' time. It added that these had been conducted as part of a bolstered fight against drug cheats.
The International Olympic Committee (IOC) said the 31 athletes in question could be banned from competing in Brazil after the re-tests of their samples collected at the 2008 summer Games in Beijing turned up positive. The samples had been stored at the IOC laboratory in Lausanne, Switzerland, and were analyzed using enhanced methods of testing.
The athletes come from 12 countries and compete in six different sports, the IOC said on its website on Tuesday.
"The Executive Board of the IOC today agreed unanimously to initiate proceedings immediately, with the 12 NOCs [National Olympic Committees] 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."
It said the positive results came from a total of 454 doping samples re-tested in conjunction with the World Anti-Doping Agency. It said a further 250 samples taken at the 2012 London summer Olympics would also be tested.
"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."
The news of the positive re-tests from the Beijing Games is just the latest doping scandal to hit the sporting world in recent months.
Last November, the International Association of Athletics Federations (IAAF), provisionally suspended Russia following a report by a WADA independent commission, which found evidence of state-sponsored doping in Russian athletics.
The IAAF is to rule on whether Russian track-and-field athletes will be allowed to participate in the Rio Games at an extraordinary council meeting next month.
US looking into Russian doping reports
The US is also investigating claims that top Russian athletes were taking part in the state-sponsored doping program, the New York Times reported on Tuesday.
Prosecutors were believed to be pursuing conspiracy and fraud charges, the report said, citing two unnamed sources.
The US attorneys were allegedly scrutinizing Russian officials, athletes, coaches, doping investigators and any others who may have benefited from the presumed scheme.
However, the investigators would need to establish a link between the alleged scandal and the US before the charges could be filed.
Kenya on 'monitoring list'
Meanwhile, athletes from Kenya, which has seen a surge in doping cases in recent years, are expected to be allowed to compete at Rio - despite the fact that WADA suspended the country's anti-doping body last week.
The IAAF placed Kenya on a "monitoring list" two months ago, where it is to remain until the end of the year.
"During the monitoring process ... Kenyan athletes remain eligible to compete nationally and internationally," the IAAF said.
pfd/msh (Reuters, dpa)