Teenage Chinese swimmer Ye Shiwen is not a drug cheat, the British Olympic Association's chairman has said, after a US coach cast doubt on her world record-breaking swim.
Lord Colin Moynihan said Ye, 16, had passed drug tests, was "clean" and deserved recognition for her talent.
Ye smashed her personal best by at least five seconds in the 400m Medley.
Senior US coach John Leonard said her performance was disturbing and hinted that doping could have been involved.
Mr Leonard, executive director of the World Swimming Coaches Association, said the performance reminded him of the East German women swimmers in the 1980s, who were doping on a systematic basis.
"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.
But Ye, who will compete later in the 200m Medley final, has strongly defended herself and denied that she would ever use banned substances.
China's anti-doping chief has said Chinese athletes have undergone nearly 100 drugs tests since arriving in London, and that not a single Chinese athlete had tested positive.
Former swimmers, other Olympic champions and experts have all given their support to Ye.
Lord Moynihan told a news conference that the World Anti-Doping Agency (Wada) was "on top of the game".
"She's been through Wada's programme and she's clean. That's the end of the story. Ye Shiwen deserves recognition for her talent," he said.
International Olympic Committee (IOC) officials also insisted that if there were any drugs cheats, they would be caught .
Dick Pound, an IOC member and former president of Wada, told the BBC that Ye's performance was "surprising".
"Nobody's ever seen somebody swim that fast before. You expect that to be an occurrence at an Olympic Games," he said.
"The problem is - with all the suspicions about drug use throughout sport - when something is really, really spectacular, the first thing you do after you see that performance is say, 'hmm, I wonder'."
Mr Pound added: "It's a fact of life that people say, 'wasn't that magnificent, was it too magnificent?'"
The US Olympic Committee (USOC) has distanced itself from Mr Leonard's comments - its spokesman, Patrick Sandusky, told the BBC the Americans were trying to smooth things over with the Chinese.
A member of USOC's international relations team is expected to make contact with the Chinese Olympic Committee later on Tuesday, says the BBC's David Bond.
"We are reaching out to the Chinese and their Olympic committee to ensure they know that this gentleman [Mr Leonard] is not part of our delegation, USA Swimming or the US Olympic Committee," the USOC member said.
"His comments are an independent view and not from us."
Earlier, former British Olympic champion Adrian Moorhouse said the rumours appeared to be a case of sour grapes.
"The Chinese might have just found this really talented kid, who can work really hard, has the perfect shape and can cope with all the pressure thrown at her," he told the BBC.
Suspicions over Ye's performance were heightened when it was pointed out that in the final 50m of her race she swam faster than the winner of the men's race, Ryan Lochte.
However, her world-record time was still 23 seconds slower than Lochte's winning time, and several of Lochte's competitors were faster than her over the final 50m.
Chinese officials also pointed out that Lochte was easing to the finish, comfortably in the lead, whereas Ye had to battle to the end to take first place.
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 the World Championships in Australia.
Chinese officials insist they have cleaned up the sport, but earlier this year another 16-year-old swimming prodigy, Li Zhesi, failed a drug test.
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.