Chinese runners Zheng Yongji and Li Huiquan were both expelled from the fifth Chinese City Games at the weekend after they tested positive for the blood-boosting drug EPO (erythropoietin).

The Chinese results are the latest in a series of positive tests which have rocked track and field in Europe and north America and could leave next year's Athens Olympics missing a host of banned big-name athletes.

It also follows the discovery of the previously undetectable designer steroid tetrahydrogestrinone (THG).

Shi Kangcheng, director of the Anti-Doping Office under the State General Administration of Sports, told official Chinese news agency Xinhua that the samples from the two Chinese athletes had revealed traces of EPO in both urine and blood tests.

"The tests were conducted on October 21 and 22 respectively," Shi told the agency. "And the A samples have returned positive results for EPO.

"According to the rules, they are expelled from the games and face further punishment."

PUNISHING OFFENDERS

Zheng Yongji finished second in group qualification for the men's 3000 metre steeplechase while 18-year-old Li Huiquan topped his group in the men's 800 metres qualification on Saturday morning.

Shi reiterated China's "three stricts" policy against doping, namely strict on prohibiting the use of drugs, strict on doping tests and strict on punishing the offenders.

Athletics officials are scrambling to defend its credibility less than a year before the Athens Olympics, after a series of positive tests have left the Games' central sport tarnished and discredited.

Four U.S. athletes have tested positive for THG, American officials have said, as has Britain's European 100 metres champion Dwain Chambers, who said through his lawyer that if he did take the drug it was unwittingly.

On Saturday U.S. Olympic 4x400 metres relay gold medallist Calvin Harrison told Reuters he has tested positive for the banned stimulant modafinil.

"I did have modafinil in my system," Harrison said. "However, I am not in the least advocating the taking of any illegal substances because I strongly believe in fair play."

ILLEGAL SUBSTANCE

Harrison said he was given the substance by a coach in California. But the athlete stressed he had never been given an illegal substance by his current coach, Trevor Graham.

Modafinil is the same stimulant that double world sprint champion Kelli White of the U.S. tested positive for at the world championships in Paris in August.

While not specified by name on the banned list, modafinil is covered under the stimulants category of "related substances", the International Association of Athletics Federations (IAAF) has said.

White has said she was prescribed the drug for a sleeping disorder. Her case is now under review by the U.S. Anti-Doping Agency and if she is found to have committed a doping offence, she could lose her Paris gold medals.

Athletics' global ruling body, the IAAF, said last week it would re-test samples taken at August's world championships in Paris to look for THG.

Details of how to detect THG, a steroid tweaked by chemists to evade detection under normal test conditions, have been distributed to 30 International Olympic Committee (IOC) accredited laboratories worldwide and a number of national federations have committed to re-testing stored samples.