Chris Froome has been informed that he failed a drugs test during the Vuelta a España stage race in September, the International Cycling Union (UCI) confirmed on Wednesday.
The British Team Sky rider had twice the permitted level of legal asthma drug Salbutamol in his body.
The UCI has asked Froome to provide more information but has not suspended him. Team Sky said Froome had suffered from asthma since childhood.
"During the final week of the Vuelta, Chris experienced acute asthma symptoms. On the advice of the Team Sky doctor, he used an increased dosage of Salbutamol [still within the permissible doses] in the run-up to the 7 September urine test."
Froome: "I know exactly what the rules are"
Thank you for all the messages of support this morning. I am confident that we will get to the bottom of this. Unfortunately I can't share any more information than I already have until the enquiry is complete.— Chris Froome (@chrisfroome) 13 December 2017
The team said Froome had declared his use of the medication, adding: "The notification of the test finding does not mean that any rule has been broken."
"I use an inhaler to manage my symptoms [always within the permissible limits] and I know for sure that I will be tested every day I wear the race leader's jersey.
"My asthma got worse at the Vuelta so I followed the team doctor's advice to increase my Salbutamol dosage. As always, I took the greatest care to ensure that I did not use more than the permissible dose."
The UCI said in a statement: "The analysis of the B sample has confirmed the results of the rider’s A sample and the proceedings are being conducted in line with the UCI Anti-Doping Rules.”