It is not known whether the retests affect medal winners from Beijing
Six Olympic athletes have been found positive for doping in retesting of Beijing samples, the International Olympic Committee (IOC) has announced.
The IOC says a total of seven positive tests involving six athletes came back positive for Cera, an advanced version of endurance-enhancing hormone EPO.
No athletes or sports involved in the positive tests have been revealed.
British Olympic chiefs believe no members of Team GB are among the six athletes caught out by the IOC.
The British Olympic Association has not been contacted by the IOC, which says the athletes in question have been notified through their national Olympic committees.
The Italian Olympic committee has confirmed one of those involved is an unnamed male Italian athlete.
US Olympic Committee spokesman Darryl Seibel said the federation had not received notification from the IOC of any adverse findings involving a US athlete.
"Unless we hear otherwise, we are treating no news as good news," Seibel told the Associated Press.
The vast majority of athletes do not seek an unfair advantage. We intend to do all we can to ensure that they have a fair environment for competition
Arne Ljungqvist, chairman of the IOC medical commission
The IOC tested 948 samples from Beijing after new tests for Cera and insulin became available after the Olympics.
BBC Radio 5 Live's Gordon Farquhar said the offenders "are likely to be competitors from the endurance disciplines of swimming, cycling, rowing and athletics."
Arne Ljungqvist, chairman of the IOC medical commission, said: "The further analysis of the Beijing samples that we conducted should send a clear message that cheats can never assume that they have avoided detection.
"The vast majority of athletes do not seek an unfair advantage. We intend to do all we can to ensure that they have a fair environment for competition."
Pending any disciplinary action by the IOC, national and international bodies are free to impose provisional suspensions of athletes, the IOC said.
Any athletes found guilty of doping face being disqualified from the Olympics and stripped of any medals.