WORLD athletics was facing its greatest crisis last night after Britain's star sprinter Dwain Chambers tested positive for a "designer" steroid.

Chambers, one of Britain's main hopes for an Olympic gold medal next year, vehemently denied doing anything wrong.

But his solicitor confirmed that a sample provided in August had tested positive for the banned substance tetrahydrogestrinone (THG).

Officials have tested scores of samples since the existence of the steroid - which has been altered to avoid conventional detection - was discovered recently.

Scientists have now found a way of testing for the substance, and hundreds of samples that had previously been given the all-clear are now being taken out of cold storage across the globe for retrospective checks.

Athletics is having to face up to the nightmare prospect of some of the biggest names in the sport being caught up in the scandal with less than a year to go before the Olympics.

So far only one other athlete - American shot put champion Kevin Toth - has been publicly named.

But sources confirmed last night that another 20 athletes, mainly US stars, have already tested positive.

Terry Madden, chief executive of the US Anti-Doping Agency, described the scandal as "an international doping conspiracy".

Athletics' governing body fears next summer's Olympics in Athens - billed as a return to traditional values - could prove a massive turn-off for TV viewers if some of the biggest names are banned.

It could also spark a revolt by sponsors who no longer wish to be associated with a tainted sport costing athletics millions at a stroke.

If a second sample from Chambers comes back positive, the 26-year-old Londoner faces a minimum two-year ban and may even be banished for life.

Last night, he vowed to fight any suggestion that he wilfully tried to deceive the authorities.

His solicitor, Graham Shear, said Chambers had never heard of THG and was "categorically assured" any supplements given to him by a nutritionist were within the rules.

Mr Shear confirmed Chambers, as part of his training, had been referred to nutritionist Victor Conte, at the Balco Laboratories in California.

The Balco lab is being investigated by US authorities for allegedly receiving millions of dollars in return for undetectable performance-enhancing drugs.

He said Mr Conte was known to have worked with many world-class athletes and was fully aware of the international rules on substance use.

Mr Shear said: "Dwain had never heard of this substance and he immediately challenged Mr Conte on this finding and was categorically assured that all supplements he had been given were within the IAAF (International Association of Athletics Federations) rules."

He added: "While the IAAF and UK Athletics procedure is still to provide any conclusive decision, my client wants to clarify that he will not accept nor tolerate any accusations or implications that this was a wilful or calculated attempt on his behalf to deceive the authorities.

"In his eight years in international athletics he has never been tempted to succumb to illegal methods of enhancing a performance."

Mr Shear said a second sample from the same time had yet to be tested and stressed Chambers would co-operate fully with any investigation.

THG only came to light when a drug whistleblower sent a syringe containing the substance to the US Anti-Doping Agency.

Yesterday, the IAAF announced it was retesting all 400 samples taken from competitors at the World Championships in Paris in August.

Mr Conte denies that any supplements he supplied to Chambers or other athletes contained any illegal substances.

Athletics coach Mike Whittingham said: "If these reports prove to be true it will be a sad day for Dwain Chambers and for British athletics."

David Moorcroft, chief executive of UK Athletics, said the sport was prepared to accept the consequences of the retesting because it was the most vigilant when it came to drug-testing.