Australia's Michael Rogers has been provisionally suspended after Clenbuterol was found in a urine sample he gave at October's Japan Cup, which he won.
The 33-year-old Saxo-Tinkoff rider, who is a three-time world time trial champion, will remain banned until his national federation has determined whether or not he has broken anti-doping rules.
Rogers won the one-day Japan Cup ahead of Garmin-Sharp's Jack Bauer and Lampre-Merida's Damiano Cunego. It was the only victory of his first season with Saxo-Tinkoff.
A statement from the International Cycling Union said: "The UCI advised Australian rider Michael Rogers that he is provisionally suspended.
"The decision to provisionally suspend this rider was made in response to a report from the WADA-accredited laboratory in Tokyo indicating an adverse analytical finding of Clenbuterol in a urine sample collected from him in a test during the Japan Cup Cycle Road Race on 20 October 2013.
"Mr Rogers has the right to request and attend the analysis of his B sample."
Saxo-Tinkoff later released a statement which said that Rogers believes contaminated food may have caused him to test positive.
The statement read: "The Australian explained to the team management that he never ingested the substance knowingly nor deliberately and fears that the adverse analytical finding [originates] from a contaminated food source."
Breyne also positive
Belgian rider Jonathan Breyne, 22, has also been provisionally suspended after testing positive for Clenbuterol at the Tour of Taihu Lake in China on November 5.
His national federation will also now convene a panel to decide if he has committed an anti-doping offence.
The news of the positive tests comes just a day after it emerged Britain's Jonathan Tiernan-Locke faces disciplinary proceedings over irregularities in his biological passport.