Toon Aerts returned a positive test for a metabolite of letrozole, which is usually a drug prescribed to treat breast cancer ©Getty Images

Triple cyclo-cross world medallist Toon Aerts has vowed to clear his name after testing positive for a metabolite of letrozole, a banned substance usually used to treat breast cancer.

The Belgian vowed to "do everything I can to prove my innocence" after his A sample from an out-of-competition test on January 19 was found to contain the metabolite.

Aerts is set to have his B sample analysed, and has not been provisionally suspended.

The International Cycling Union (UCI) confirmed the positive test, but as letrozole appears as a specified substance on the World Anti-Doping Agency Prohibited List, a provisional suspension is not mandatory.

The UCI said it "will not comment any further on the matter", and only moved to confirm the adverse analytical finding because Aerts himself had made it public.

Aerts, in his statement, claimed to be "in the dark as to how this could have come about" and that letrozole was a "product that I had never heard of until yesterday".

Letrozole is an aromatase inhibitor and lowers estrogen levels.

Toon Aerts is a three-time World Championship bronze medallist ©Getty Images
Toon Aerts is a three-time World Championship bronze medallist ©Getty Images

Italian tennis player Sara Errani is among other athletes to have tested positive for letrozole.

Errani - who claimed to have ingested the substance through a family meal prepared by her mother, who was taking letrozole - was banned for 10 months.

In 2020, boxer Virginia Fuchs was cleared of a doping offence when the United States Anti-Doping Agency accepted that a positive test for metabolites of letrozole and GW1516 was caused by sexual transmission, with Fuchs' male partner using therapeutic doses of both letrozole and GW1516.

Aerts is one of the best cyclo-cross riders in the world.

The 28-year-old has twice won the overall UCI Cyclo-cross World Cup title, is a three-time bronze medallist at the World Championships and is a former European champion.

At last month's World Championships in Fayetteville in the United States, Aerts finished sixth in the men's race.