The 27-year-old Spanish rider, who claimed his third Tour de France crown when he won this year's race, was tested on July 21, but is claiming that the result of the test is a result of food contamination.
Contador is set to hold a press conference later today, but his spokesperson released a statement earlier which read: "Alberto Contador is affected by a doping control at the last Tour de France on July 21, where it was found the substance clenbuterol.
"From the time of the first communication from the UCI (cycling's world governing body), August 24, Alberto Contador alleged food contamination as the only possible explanation of what happened and has put it in the hands of the cycling authorities, in the confidence that they can clarify this very serious problem, which is now made public.
"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, 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, who rides for the Astana team but will switch to Bjarne Riis's Saxo Bank-SunGard outfit in 2011, pulled out of the Dempsey Challenge in the US earlier this week after also electing to skip the UCI Road World Championships, which are underway in Melbourne
A notice on the Dempsey Challenge website read: "Due to a personal matter, Alberto Contador will be unable to travel to the United States and participate in the Dempsey Challenge. Alberto apologises to his fans and all involved with the Dempsey Challenge and wishes everyone an enjoyable weekend."
Contador cited a desire to better prepare for next season as his reason for missing the world championships in Australia.
