While refusing to guarantee that this year's Olympics would be drug free, the World Anti Doping Agency head was adamant they would be the cleanest ever despite doubts raised over the effectiveness of drug tests.

"One has to recognise the question of doping in sport has been around now for a long time," Fahey told the Australian Broadcasting Corporation. "There's been evidence that at successive games it's occurred.

"(But) I can give this guarantee: there's a far greater likelihood that anybody cheating or attempting to cheat in the Beijing Games will be caught than in any other time of our history," he told ABC radio.

Fahey also said that Italian rider Riccardo Ricco tested positive at the Tour de France after a secret molecule was planted in the blood booster EPO during its manufacture in a bid to crack down on drug cheating in sports.

And he questioned suggestions that his agency's EPO (erythropoietin) tests had been proven fatally flawed by independent studies, including a report that claimed WADA laboratories were classing positive EPO tests as negatives and vowed to keep up a vigorous pursuit of drug cheats.

"We do not ignore such evidence (but) it is questioned," Fahey said, adding experts had cast serious doubts on the external research.