PARIS (AFP) - Four-time Tour de France winner Chris Froome failed a drug test during the Vuelta e Espana race in September, cycling's governing body UCI said 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. He risks being stripped of his first Spanish Grand Tour crown.
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 Sept 7 urine test."
The team said Froome, who won the Tour in 2013 and consecutively from 2015 to 2017, had declared his use of the medication, adding: "The notification of the test finding does not mean that any rule has been broken."
Froome said the UCI was "absolutely right" to scrutinise the test results. He said that during the race he had "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," he said. "Together with the team, I will provide whatever information it requires," he added.
Froome was notified of the test result on Sept 20, the day he finished third in the world time-trial championship in Bergen, Norway. He has not competed since then, but has announced his intention to try to win the three big tours, in France, Spain and Italy, in 2018.
The test will raise new questions about British cycling following the scandal surrounding the only previous British Tour winner, Bradley Wiggins, over his use of so-called therapeutic use exemptions (TUEs).
It emerged that Wiggins received TUEs in order to take a corticosteroid before his three biggest races in 2011, 2012 and 2013, including his 2012 Tour win. Wiggins and Sky have repeatedly denied any wrongdoing, saying the drug was prescribed to treat a longstanding pollen allergy.