A statement by the sport's governing body, the International Cycling Union (UCI), said Contador tested positive for a "very small concentration" of clenbuterol, a banned anabolic agent, during the second rest day of the 2010 Tour de France. He has been provisionally suspended.
Contador, who is regarded as one of the sport's greatest ever riders and won his third Tour this year, vehemently denies the doping allegations, telling a news conference that contaminated meat was to blame for the positive result.
"It tugged at my heartstring when I heard the news (about Contador). Such scandals don't do the sport any good. Especially for the sponsors, who try to avoid such bad publicity," Francis Lafargue, head of communications at Team Caisse d'Epargne, told Reuters.
The UCI also revealed that Mosquera had tested positive for a banned substance which helps increase blood volume delivering oxygen to the body.
Among a host of high-profile offenders, US cyclist Floyd Landis was stripped of his 2006 Tour de France title for a positive drugs test.
