The IOC says 31 athletes in six sports have tested positive in reanalysis of their doping samples from the 2008 Beijing Olympic Games.
The International Olympic Committee (IOC) says it has opened disciplinary proceedings against the unidentified athletes from 12 countries.
The samples had been stored at the IOC laboratory in Lausanne. They were retested using enhanced methods on athletes who were expecting to compete in August at the Olympics in Rio de Janeiro.
The IOC says "all those athletes infringing anti-doping rules will be banned from competing" in Rio.
The IOC says the results of 250 retests from the 2012 London Olympics will "come shortly."
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The committee will also undertake a "wider retesting" of medallists from Beijing and London.
