Contador, a national hero in his native Spain, is scheduled to speak about his positive test Thursday at a news conference in Pinto, his hometown. Though he never failed a drug test before, Contador in 2006 was initially implicated in a large-scale blood-doping ring in Spain. Later, the International Cycling Union cleared him of any wrongdoing.

At last year’s Tour, Contador reportedly set a speed record during one climb, prompting questions  including ones from the three-time winner Greg LeMond  about whether he was clean. In response, Contador said he had never taken performance-enhancing drugs and was against the doping that had weakened his sport’s credibility.

One of the biggest doping controversies hit the sport in 2006, when Floyd Landis won the Tour, then was stripped of the title after testing positive for synthetic testosterone. Landis had insisted that he had never used performance-enhancing drugs, but this spring he admitted using them.

If Contador is stripped of his Tour title, he will become the second rider to lose the title because of doping. And it looks as if he will have an uphill battle to retain that title.

The presence of the drug in an athlete’s system means that the athlete is disqualified from the day of the positive test forward, according to World Anti-Doping Agency rules. But if it is proven that the ingestion of the drug was unintentional and that the athlete was not at fault, that ban could be reduced to zero in rare cases, antidoping experts say.

Traces of clenbuterol have been found in nutritional supplements. The swimmer Jessica Hardy missed the 2008 Olympics after testing positive for the drug; she said she had taken a contaminated supplement. An arbitration panel later ruled that her claim was true and reduced her two-year ban to one year.