Top British sprinter Dwain Chambers has tested positive for the steroid tetrahydrogestrinone (THG).
According to The Guardian newspaper, traces of the banned drug were found in a urine sample given by the athlete in an out-of-competition test in Saarbrucken, Germany.
A second, or B test on the same sample has yet to take place.
A spokesman for Chambers' management company, Stellar, said: "It would be inappropriate to comment at the moment."
If found guilty of using banned substances such as THG, he faces a minimum two-year ban from international competition.
Under British Olympic Association rules, Chambers would also be barred from Olympic participation for life.
DOPE TEST PROCEDURE FOR OVERSEAS TESTS
Athlete's sample divided into two parts - A and B
If A sample tests positive, B sample sent for testing
If B positive, IAAF decide if there is a case to answer
If so, matter referred to athlete's national agency
(UK Athletics in UK)
Independent committee appointed by UK Athletics
Athlete presents defence to committee at closed hearing
Committee gives verdict - and sentence if athlete found guilty
His coach, the US-based Ukrainian Remi Korchemny refused to accept that his athlete would test positive for an illegal steroid.
He told the San Jose Mercury News: "This is not something the public should just accept.
"Can you say someone is guilty without a judge and jury? You are supposed to be innocent until proven guilty."
Lyn Davies, the president of UK Athletics and a former Olympic long jump champion, declined to comment directly about the Chambers case.
But he told the BBC: "UK Athletics is very vigilant and does everything possible to educate athletes and warn them [about the consequences of using drugs].
"But if you become Olympic champion or world champion today, it means that you become a millionaire.
"Obviously the motivations and the temptations are understandable - but it's very disappointing."
DWAIN CHAMBERS FACTFILE
European 100m champion
Joint GB 100m record holder with Linford Christie: 9.87secs
Won 100m bronze at 1999 World Championships
Disappointing 4th in 100m at 2003 Worlds in Paris
Another former Olympic champion, Lord Coe, a member of the IAAF council and deputy chairman of London's 2012 Olympic bid, admitted: "Some people will always keep trying to cheat.
"We have probably opened a can of worms but I would rather have that embarrassment than see the decline of the sport," he told BBC Radio Five Live.
THG came to the attention of the athletics authorities when an anonymous coach turned in a used syringe containing the substance.
Until last week it was thought to be undetectable by normal dope tests.
But is believed that up to 20 American athletes tested positive for the drug at June's US championships.
Athletics' world governing body the IAAF has said that anyone found guilty would face serious consequences.
Under American athletics' rules, the athletes in question have not been identified pending analysis of their B samples.
Anabolic steroids can improve the body's capacity to train and compete at the highest level.
They reduce the fatigue associated with training and the time required to recover after physical exertion.
They also promote the development of muscle tissue in the body, with an associated increase in strength and power.