Garzelli won Friday's fifth stage of the Giro d'Italia
Mapei's Stefano Garzelli has tested positive for performance-enhancing drugs.
Italian Garzelli led the Giro d'Italia by 43 seconds after the fifth stage but competed the sixth stage on Saturday.
"I wanted to quit the Giro but my team asked me to stay," said Garzelli.
Mapei confirmed on Saturday that Garzelli returned a positive sample when he won the second stage of the Giro at Ans in Belgium, and that they were awaiting the second test.
Garzelli reportedly tested positive for the diuretic Probenecid, which is considered a drug-masking product.
Speaking at a news conference, the Mapei team said they were surprised by the results of the test on Garzelli.
"It's crazy that he would take something like this because it's certain to show up in a drugs test," said team press officer Alessandro Tegner.
Team manager Aldo Sassi added: "We don't want to say there's been a plot against us. We are just reporting the facts that we know.
"We've asked the UCI (International Cycling Union) to test all our riders to see if they're positive for the same product."
Garzelli was close to tears at the conference.
"My conscience is clear," he said. "I have been preparing for the Giro since last year. I would be an idiot to ruin the race, my career and my life for something like this."
Race officials have also revealed that two other riders have returned positive tests for the new endurance-boosting drug Darbepoetin, or Nesp.
Race favourite
Garzelli, winner the Tour of Italy in 2000, was among the race favourites having finished second in the Liege-Bastogne-Liege last month.
The tests are a blow for the race which has been bidding to emerge from last year's drug-tainted Giro when Italian police raided the hotel rooms of all 20 teams and found a variety of doping products.
It led to the sacking of Dario Frigo by his Fassa Bortolo team after being found in possession of illegal drugs. The Italian had been lying second overall.
Is cycling damaged beyond repair?
Overall standings:
1. Stefano Garzelli (Ita) 26hr 21min 28sec
2. Francesco
Casagrande (Ita) at 43 secs
3. Gilberto Simoni (Ita) 1min
4. Santiago
Perez (Spa) 1min 3sec
5. Wladimir Belli (Ita) 1min 6sec
6.
Juan Carlos Dominguez (Spa) 1min 10sec
7. Fernando Escartin (Spa)
1min 11sec
8. Tyler Hamilton (USA) 1min 13sec
9. Cadel Evans (Aus)
1min 14sec
10. Davide Rebellin (Ita) 1min 19sec