The disease of illicit drug use continues to blight athletics after two more competitors tested positive for banned substances at a meet in China.

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 21st and 22nd respectively," Shi said. "And the A samples have returned positive results for EPO. According to the rules, they are expelled from the games and face further punishment."

Zheng Yongji finished second in group qualification for the men's 3,000-metre steeplechase while 18-year-old Li Huiquan topped his group in the men's 800 metres qualification on Saturday morning.

Athletics officials are scrambling to defend the sport's 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 USA athletes have tested positive for THG, American officials have said, as has Britain's European 100 metres champion Dwain Chambers.

And US Olympic 4x400 metres relay gold medallist Calvin Harrison has admitted he tested positive for the banned stimulant modafinil. "I did have modafinil in my system," he said. "However, I am not in the least advocating the taking of any illegal substances because I strongly believe in fair play."

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

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.