PARIS — Three-time Tour de France champion Alberto Contador tested positive for a banned drug while winning this year's race and has been suspended by cycling's governing body, dealing a potentially crushing blow to his sport's efforts to shake off its doping-soiled past.

Contador blamed contaminated food for the test result.

A World Anti-Doping Agency-accredited lab in Cologne, Germany, found a "very small concentration" of the muscle-building and fat-burning drug clenbuterol in the Spanish rider's sample on July 21 at the Tour, according to a statement from the UCI.

But the amount was "400 time(s) less than what the antidoping laboratories accredited by WADA must be able to detect," the cycling organization said.

Both Contador's A and B samples tested positive and the cyclist has been "formally and provisionally suspended," the UCI said.

With seven-time Tour champion Lance Armstrong now back in retirement, Contador is cycling's biggest star, so it could be devastating for the sport if he is proved to have cheated.

Having invested millions of dollars in recent years in what is widely regarded as the one of the most stringent anti-doping regimes anywhere, cycling authorities hoped to be turning the corner on widespread doping by riders that had long made a mockery of the sport and repeatedly sullied the Tour, its showpiece race. Although just 27, Contador is already the greatest rider of his generation. His victories at the Tour starting in 2007 and at other major races were seen as a possible break from cycling's dirty past.

The UCI's statement gave no indication of whether Contador will be stripped of his latest Tour title or be banned.

Jacinto Vidarte, Contador's publicist, released a statement saying the cyclist insists food contamination is the only possible explanation.

"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 was scheduled to hold a news conference today in Pinto, his childhood home in Spain.