The International Cycling Union on Wednesday said samples from Contador taken during this year's Tour de France came back positive and suspending the three-time Tour champion.
Contador's press agent said the banned substance in question was clenbuterol, which is used to treat breathing problems such as asthma,
Contador, at a news conference in Spain, said the positive test results were due to a "clear case of food contamination."
"It's such a minimal amount that it's only possible to come from a food contamination," Contador said. "And in terms of performance it would not be any use at all -- any expert can confirm this."
The International Cycling Union said tests indicated "very small" concentrations of clenbuterol but still provisionally suspended Contador. There was no indication if Contador would have his 2010 Tour de France title take from him.
The test samples were taken July 21. Contador had taken the overall Tour de France lead two days earlier and four days after the test won the Tour de France by 39 seconds over Andy Schleck.
Since winning the 2010 Tour de France, Contador has left the Astana cycling team and signed on with Saxo Bank. He won his first Tour de France in 2007 and also won in 2009 in addition to this year's title.