Britain's most successful road cyclist Chris Froome is facing questions after a urine test revealed abnormal levels of an asthma drug.
The four-time Tour de France winner provided a sample during the 2017 Vuelta race with a concentration of Salbutamol that was double the threshold of the World Anti-Doping Agency (WADA).
Froome, who has not been provisionally suspended, vowed to cooperate with cycling's world governing body, the UCI.
"The UCI is absolutely right to examine test results and, together with the team, I will provide whatever information it requires," Froome said in a statement.
"It is well known that I have asthma and I know exactly what the rules are.
"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."
Froome, 32, had followed up his fourth Tour de France title in June by winning the Vuelta in Spain in September.
The urine test, conducted on 7 September, revealed a concentration of Salbutamol of 2,000 nanograms - twice the WADA threshold of 1,000.
Italian cyclist Alessandro Petacchi was handed a 12-month ban in 2007 for excessive use of Salbutamol during the Giro d'Italia and stripped of his five stage wins.
Team Sky said Froome experienced "acute asthma symptoms" during the final week of Vuelta and increased his dosage of Salbutamol, within permissible limits, on doctor's advice.
The "use of permissible dosages of Salbutamol can sometimes result in elevated urinary concentrations, which require explanation", Team Sky said.
Team Sky Principal Sir Dave Brailsford said: "There are complex medical and physiological issues which affect the metabolism and excretion of Salbutamol.
"We're committed to establishing the facts and understanding exactly what happened on this occasion.