Toon Aerts, the 2016 European cyclo-cross champion and three-time World Championships bronze medallist, has tested positive for a banned substance.
Confirming the positive out-of-competition test in a statement, Aerts strongly denied doping and claimed that he "did not know" how the substance had entered his body.
While the Belgian awaits the results of his B sample taken on 19 January, the 28-year-old and his team have agreed to end his season.
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"Yesterday my world was turned upside down," the Baloise Trek Lions rider said. " received a message that an abnormal result was found in my urine sample that was taken during an out-of-competition check at my home on January 19th.
"I am currently in the dark as to how this could have happened. The product letrozole metabolite was found in my urine sample - A product I had never heard of until yesterday and I don't know how it got into my body.
"At the moment my mind is not on races and I have decided in consultation with the team to end my season until there is more clarity in this matter."
Aerts was the back to back winner of overall titles in the UCI World Cup in 2018-19 and 2019-20.
He secured a third successive world championship bronze last year, but could only finish sixth in Fayetteville last month.
Issuing a strong denial of wrongdoing, Aerts' statement continued: "Anyone who knows me at all knows that I have been against any form of doping all my career and have always done everything to set an example as an athlete.
"I will therefore do everything I can to prove my innocence and clear my name.
"Pending the analysis of the B sample and further investigations, I will not comment. I would urge everyone to respect this and to give me and my family the time and opportunity to get more clarity ourselves first."
Letrozole is in medications used to fight breast cancer in women.
According to the US Anti-Doping Agency, male athletes may use it to boost natural levels of testosterone - the substance prevents the breakdown of the hormone.
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