|Contador celebrates his third Tour de France win in Paris in July after beating Andy Schleck [GALLO/GETTY]
Tour de France champion Alberto Contador has been provisionally suspended from cycling after trace amounts of a banned substance were found in two urine samples taken during his victory this year.
The Spanish rider returned an "adverse analytical finding" of clenbuterol, a drug used to treat breathing problems, the International Cycling Union (UCI) said on Thursday.
The cyclist, who won his third Tour de France this year, was tested during a rest day on July 21.
He held a press conference in Pinto, Spain, on Thursday, blaming contaminated beef for his positive test.
"It is a clear case of food contamination," he said.
Contador said his team had complained about the meat, which was brought across the border from Spain to France during a rest day ahead of a stage in the Pyrenees mountains.
The Spaniard said he ate the meat on July 20 and again on July 21.
"Three hours after eating this beef, I had a drug test," he said.
He called the UCI's suspension of him "a true mistake".
A World Anti-Doping Authority (Wada) laboratory in Cologne, Germany, found that the second 'B' test had confirmed the presence of clenbuterol, a banned anabolic agent.
"The rider, who had already put an end to his cycling season before the result was known, was nevertheless formally and provisionally suspended as is prescribed by the World Anti-Doping Code," the UCI statement said.
"From the moment he was first informed by the UCI on August 24, Alberto Contador indicated that a contamination of food was the only possible explanation for what happened"
Jacinto Vidarte, Alberto Contador's agent
The concentration in the A test was "400 time(s) less than what the antidoping laboratories accredited by Wada must be able to detect," the statement continued.
"In view of this very small concentration and in consultation with Wada, the UCI immediately had the proper results management proceedings conducted including the analysis of B sample that confirmed the first result."
The case would require "further scientific investigation" before any conclusion could be drawn and could take some time.
"From the moment he was first informed by the UCI on August 24, Alberto Contador indicated that a contamination of food was the only possible explanation for what happened," his agent Jacinto Vidarte said in a statement carried by Spanish media earlier on Thursday.
"Since then, he has placed himself in the hands of the cycling authorities with the confidence that this extremely serious problem that has now come to light can be resolved."
"Experts consulted have also signalled that food contamination was the origin of the case, above all taking into account the quantity of (doping) tests Alberto Contador took during the Tour de France.
Intent 'ruled out'
The statement went on: "(This) enabled the exact moment of the appearance of the substance to be determined as well as the very small amount detected, which rules out any other origin or intent."
Although just 27, Contador is already the greatest rider of his generation.
His victories at the Tour and elsewhere were seen as a possible break from cycling's dirty past.
If he is proved to have cheated, cycling would suffer a devastating blow, having invested millions of dollars in what is widely regarded as the one of the most stringent anti-doping regimes anywhere.
Clenbuterol is used to treat breathing disorders, rising blood pressure and for oxygen transportation, and has led to bans for a number of cyclists in the past.
China's first professional cyclist Li Fuyu, a former RadioShack teammate of Lance Armstrong, was provisionally banned by the UCI in April after returning a positive test for the anabolic agent.
Ruper Guinness, a cycling writer for the Sydney Morning Herald, told Al Jazeera that the news was "a major blow".
"Many people have been left reeling at the news," he said.
"There is still the question mark of why the substance was in him. It's another major blow for the sport.
"There's going to be ramifications but at least it shows the tests are capable of catching cheats."
Wada director general David Howman told the Associated Press news agency on Thursday that testing positive for even the most minute amounts of clenbuterol could be enough to sanction an athlete, although he refused to discuss the specifics of Contador's case.
"There is no such thing as a limit where you don't have to prosecute cases," he said.
"This is not a substance that has a threshold. Once the lab records an adverse finding, it's an adverse finding and it has to be followed up.
"Clenbuterol is a substance that has been used for over 20 to 30 years.
"It is not anything new. Nobody has ever suggested it is something you can take inadvertently."
In July, Contador won the Tour de France for the third time in four years, beating Andy Schleck of Luxembourg by 39 seconds.
Having won all three Grand Tours of France, Italy and Spain – something seven-time Tour de France champion Lance Armstrong never achieved – he has been on course to become one of cycling's all-time greats.
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.