Cycling's governing body says a "very small concentration" of a banned substance was found in the urine of Tour de France winner Alberto Contador, but that he has been suspended.
The UCI says a lab in Germany found clenbuterol in Contador's urine sample on July 21 at the Tour de France. But the amount was only 50 picograms, which the UCI says "is 400 time(s) less than what the antidoping laboratories accredited by the World Anti-Doping Agency's must be able to detect."
It says that both Contador's A and B samples tested positive and that Contador has been "formally and provisionally suspended."
It says that the case is still being investigated and that this may take some time.
The UCI released its statement in the early hours of Thursday.
Jacinto Vidarte, Contador's publicist, said Contador has blamed food contamination as the only possible explanation for the test.
"The experts consulted so far have agreed also that this is a food contamination case, especially considering the number of tests passed by Alberto Contador during the Tour de France," Vidarte said in the release, "making it possible to define precisely both the time the emergence of the substance as the tiny amount detected, ruling out any other source or intentionality."
Contador will hold a news conference on Thursday in Pinto, Spain.
Contador was first made aware of the positive test on Aug. 24, according to statement, and has provided his explanation to the proper authorities.
In July, Contador won the Tour de France for the third time in four years, beating Andy Schleck of Luxembourg by 39 seconds.
If Tour officials strip Contador of his title, he would be just the second cyclist so punished. The first was American Floyd Landis, who was stripped of his 2006 Tour title after a positive test. For years he denied doping but admitted this spring that he used performance-enhancing drugs. In doing so, he accused seven-time Tour winner Lance Armstrong and others of systematic drug use.
Clenbuterol is a synthetic bronchodilator often prescribed to asthma sufferers. The drug is on the WADA's banned substances list and its use would mean a two-year ban from the sport.
American swimmer Jessica Hardy tested positive for clenbuterol at the U.S. trials in July 2008. She served a one-year suspension that ended last summer. The Court of Arbitration for Sport accepted her explanation that she had unknowingly taken the banned anabolic agent in a contaminated food supplement.
A number of athletes have been banned in recent months after using the banned drug, including Polish canoeist Adam Seroczynski, British hurdler Callum Priestley and Chinese Olympic judo champion Tong Wen.
Two cyclists also have been suspended, accused of using the drug. In May, the International Cycling Union suspended Italian cyclist Alessandro Colo after he tested positive for clenbuterol during the Tour of Mexico in April. And Chinese rider, Li Fuyu, a member of Lance Armstrong's Team RadioShack, was suspended in April after testing positive for the drug during a Belgian race.
Former New York Mets clubhouse employee Kirk Radomski admitted to distributing clenbuterol to dozens of current and former Major League Baseball players and associates in his plea deal.