Twenty-three athletes who competed at the 2012 London Olympics have tested positive for banned substances after their samples were reanalysed.
The International Olympic Committee (IOC) revealed that the 23 competitors, spanning five sports, had tested positive after new analysis of 265 samples.
The IOC said it is continuing to reanalyse the samples with the latest technology, adding that more results could come out in the next few weeks.
It will not name the athletes or the sports until second samples have been tested, a process which can take several weeks.
The athletes concerned are being informed of the test results, after which proceedings against them can begin.
The current re-testing programme targeted athletes who could be eligible to compete this summer.
Those found to have breached anti-doping rules will be banned from taking part.
"These reanalyses show, once again, our determination in the fight against doping," said IOC president Thomas Bach.
"We want to keep the dopers away from the Olympic Games in Rio de Janeiro. This is why we are acting swiftly now.
"I have already appointed a disciplinary commission, which has the full power to take all the decisions on behalf of the IOC."
It was also revealed that one more sample from the Beijing Games had shown "abnormal parameters" and would be followed up on.
The samples were re-examined after intelligence-gathering that began last August, the IOC said in a statement. It has been working with the World Anti-Doping Agency (WADA) in that period.
Last week, it announced that 31 athletes could be banned from competing at Rio de Janeiro this summer, after re-examining hundreds of samples from the 2008 Games.
Fourteen of those athletes were Russian, and Sky's Paul Kelso said it would be "a big surprise" if this latest announcement does not involve any competitors from the country.
One of the 14 Russians who tested positive was high jumper Anna Chicherova, who won bronze in 2008 and gold in 2012.
She vowed to clear her name after learning she had tested positive.
"It's the most complete shock. I can't explain how it could have happened. I was always sure what supplements and medicines I was using," she said.
The latest revelations come as a further blow to the sporting event's image.
Concerns about doping have plagued the build-up to this year's games, due to be held in Rio in August.
Russia remains under investigation after a WADA report suggested "systematic" doping was taking place in the country.
Its track and field athletes are currently suspended from competing, and the Olympic powerhouse could be without a full team at the upcoming games.
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