China's former Olympic doctor says he has long suspected Michael Phelps of using performance-enhacing drugs, but has kept silent until now because of the absence of evidence.

His attack comes in response to allegations raised by an American swim coach against 16-year-old Ye Shiwen, who shattered the women's 400m individual medley world record.

"Abnormal?", asked Chen Zhanghao, who headed the Chinese Olympic medical team in Los Angeles, Seoul and Barcelona. "America's Phelps broke seven world records! Is he normal?

"I suspect Phelps, but without evidence, I have to recognise that we should be grounded in facts," he said.

"The Americans have made many extraordinary performances but without evidence we have kept silent."

Dr Chen's comments are significant because he himself revealed his own role in Chinese sports doping in the 1980s and early 1990s.

On Tuesday he said China had cleaned up its act in the 1990s and that it would now be "impossible" for an elite Chinese athlete to get away with deliberate doping.

"Once caught, the athlete, coach and sometimes team manager are dismissed," he said. "Before these Olympics, the Chinese sports administration repeatedly stressed the importance of anti-doping. It has been raised to a question of state dignity."

Dr Chen alleged the US has been making and using sports drugs since the 1940s and continues to produce many banned substances.

"The Americans are very bad, they do a lot of evil," he said. "But they don't talk about themselves."

He said Ye Shiwen was successful because she has rare talent.

-Sydney Morning Herald