Alberto Contador Thursday appeared at a press conference to proclaim his innocence over accusations of doping in the Tour de France cycle race.

Contador this summer won the three-week long race for the third time, holding off the challenge of Luxembourg's Andy Schleck, but that victory has now been soured with the news that he tested positive for the banned substance clenbuterol, a drug used for respiratory problems.

The positive was taken on July 21, the day before Contador held off Schleck's attacks on the hardest stage of the race, which climbed the Tourmalet mountain on two occasions.

Riding for the Astana cycling team, Contador won the overall race by the narrow margin of 39 seconds ahead of Schleck.

The rider's immediate reaction to the publication of the news of his positive test was to issue a communique stating that the positive was the result of contaminated food.

"He has put the affair into the hands of cycling authorities in the confidence that this serious problem can be cleared up now that it has been made public," said the communique.

However, the rider, who has won the Tour three times, in 2007, 2009 and 2010 as well as triumphing in the 2008 Giro de Italia and Vuelta de Espana, was more outspoken this Thursday.

"It is important to send a clear message and for it to be distributed, both for my good and the good of the sport, because this is a social problem. This is the consequence of a quantity of celmbuterol in an anti doping control taken in the Tour de France on the afternoon of July 21. We are talking of a millesima (tiny) part of clembuterol -50 picograms, the equivalent of 0,00000000005 grams per milliliter of urine, or 0.05 nano-grams," he said, highlighting the tiny amount of the substance that had been found, before repeating how he had eaten some contaminated food.

"The UCI (International Cycling Union) told me of the appearance of the sample on the afternoon of August 24, on the 26th I met the UCI medical team and I explained what had happened. The UCI confirmed that it was a case of contaminated food," he said.

Contador explained how the food in question was some meat that Lopez Cerron, (the director of the cycling Tour of Castilla and Leon) had taken to France.

"He was coming to the Tour and asked if he could bring us something and the team cook, asked him to bring some really good meat. Lopez Cerron bought it in a shop in Spain, although I don't know exactly where, and on the afternoon of July 20 the meat was cooked in the team bus. We ate the meat and on the following day, the rest day, I had a blood test. Then we ate more of the meat and hours later I had a urine test; that is the day they detected the substance," he explained.

Contador also questioned the testing system and added that the minute levels that had been found in his urine were not enough to improve performance, before showing his annoyance at having to explain himself.

"I am sad and disappointed, but I can keep my head high, because you can speak loud and clear when you say the truth. I am not worried that my results will be doubted. I can defend myself pointing to all of the controls I have passed through."

"I am confident the UCI can clearly see what happened, but I would not tolerate a sanction," he said.

The 27-year-old began his professional career in 2004 in the ONCE- Eroski cycling team which had become known as Liberty Seguros - Wurth cycling team by the time it was kicked out of 2006 Tour de France as a result of the Operation Puerto doping scandal.

Contador was cleared by Spanish courts of having formed part of the widespread doping operation, but Liberty Seguros rider, Isidro Nozal, who was also cleared by the police, subsequently tested positive for use of the blood boosting product, EPO.