The head of the World Anti-Doping Agency admitted today that confidence in the Olympics had been shattered by a succession of drugs scandals involving athletes, writes Matthew Beard in Beijing.

John Fahey said unless sport cleaned up its act at the Games here, there was a danger that the public would "desert" them.

His grim warning came as Olympic chief Jacques Rogge said that if fewer than 30-40 athletes tested positive in China, this would be a victory in the war on drugs.

It emerged today that three male Russian race walkers have tested positive for the banned substance EPO and, earlier in the week, seven women from the same country - including world champion middle-distance runner Yelena Soboleva - were suspended on suspicion of supplying fake urine samples.

Fahey insisted, though, that the Beijing Games, coming 20 years after Ben Johnson failed a drugs test in Seoul, had the chance to change attitudes.

Fahey said: "There is suspicion out there in the public, they do not have the same confidence they once had. That means that we must restore sport to its very essence of the fair play concept. We must, because the public will desert any sport in time if they are not satisfied with its integrity."

Fahey believed that WADA were winning the fight against drugs in sport, even though Canadian sprinter Johnson has since been joined in a list of shamed gold-winning athletes by Britain's Linford Christie and Americans Justin Gatlin and Marion Jones.

He added: "In the blue riband event of athletics [100m], a number of offenders have been successful and I hope we do not have another event of that nature.

"Step one would be for somebody to win on merit and nothing taints it."

International Olympic Committee president Rogge said 4,500 athletes would be tested over the coming two weeks.

In Athens the IOC carried out 3,500 tests producing 26 positive results.

He added: "If we have less [than 30-40] then I will be very glad because there will have been relatively fewer positive cases which will show there has been a deterrent effect."