1 July 2012 Last updated at 06:34 GMT
Chinese Olympic swimmer Ye Shiwen denies doping
China's 16-year-old swimming star Ye Shiwen has denied taking performance-enhancing drugs after smashing a world record at the London Olympics.
She spoke out after a leading US coach described her performance in the 400m individual medley as "disturbing".
Ms Ye swam the 400m individual medley seconds faster than she ever had before and - on the last 50m - faster than the winner of the men's event.
There is no evidence against her and all medal winners are drug-tested.
On Monday, Ye Shiwen took at least five seconds off her personal best to break the world record by more than a second and win the gold medal in the individual medley.
In the last 50m she swam faster than US star Ryan Lochte in the men's event.
Commentators were stunned, including the BBC's Clare Balding who said questions would be asked about the swimmer's performance.
American coach John Leonard, executive director of the World Swimming Coaches Association, said her performance was "unbelievable" and "disturbing".
"History in our sport will tell you that every time we see something, and I will put quotation marks around this, unbelievable, history shows us that it turns out later on there was doping involved," he told the UK's Guardian newspaper.
China's swimming team was repeatedly hit by doping scandals in the 1990s.
Seven swimmers tested positive for drugs in the 1994 Asian Games, and four years later four Chinese swimmers failed pre-tournament drug tests before swimming world championships in Australia.
But Ye Shiwen denied the allegations, telling reporters: "My results come from hard work and training and I would never use any banned drugs. The Chinese people have clean hands."
The BBC's Martin Patience in Beijing says the accusations have sparked an angry reaction from Chinese internet users, who have accused other nations of jealousy.
Chinese swimming team leader Xu Qi told China's state-run Xinhua news agency that Ms Ye's result had been expected.
"To compare Ye's result with Lochte's is meaningless," he said.
"Ye was behind after 300m and she needed to try her best to win the race, but Lochte had already established the lead before the freestyle and didn't need to do his upmost."
Arne Ljungqvist, medical commission chairman for the International Olympic Committee, called the speculation sad.
"To raise suspicion immediately when you see an extraordinary performance - to me it is against the fascination of sport," he said.
All medal winners at the Olympics are drug tested. In addition, any athlete whose performance is far better than anything they have achieved before can be targeted for extra tests.
China's anti-doping chief has said that Chinese athletes have undergone nearly 100 drugs tests since arriving in London, and that not a single Chinese athlete had tested positive.
But, our correspondent says, China's national swimming team has been marred by doping incidents in the past.
During the 1990s almost a dozen of its top swimmers were banned for using performance enhancing drugs.