Froome had double the legal limit of asthma drug
Chris Froome could miss most of next season after a urine sample he gave at this year's La Vuelta was found to contain twice the permitted concentration of asthma drug Salbutamol.
The 32-year-old Team Sky rider may also lose his victory in that race - the first by a British cyclist - and be unable to defend his Tour de France title next July or attempt to win a third straight Grand Tour title at the Giro d'Italia in May.
The adverse analytical finding occurred in a routine test after the Vuelta's 18th stage on September 7 - a day that saw Froome respond to a disappointing ride the day before by stretching his lead over rival Vincenzo Nibali on the last climb.
Conducted by the Cycling Anti-Doping Foundation, the test found the concentration of Salbutamol in Froome's urine sample was 2,000 nanogrammes per millilitre (ng/mL), double the World Anti-Doping Agency's (WADA) limit of 1,000 ng/mL.
Salbutamol, which is also marketed as Ventolin, is widely used by asthma sufferers, most commonly in an inhaler, to relax the muscles in the airway.
Salbutamol is banned by WADA when taken intravenously or in pill form - as research suggests large doses administered like this can boost performance - but asthma sufferers are allowed to take up 1,600 microgrammes over 24 hours without exceeding 800mcg every 12 hours. A typical dosage, or puff, is 100 mcg.
In a statement issued by Team Sky, Froome said: "It is well known that I have asthma and I know exactly what the rules are. I use an inhaler to manage my symptoms 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.
"I took the greatest care to ensure I did not use more than the permissible dose."
Froome, who records his Salbutamol use on doping control forms, was informed of the adverse finding on September 20, the day he capped a stunning season with a bronze-medal ride in the time trial at the Road World Championships.
The UCI said Froome's B sample - athletes' anti-doping samples are split into A and B samples as a fail-safe precaution - had been analysed and it confirmed the results of the initial test. The Swiss-based body added that under its rules Froome is not subject to a mandatory suspension.
The next stage in the process will be for Froome and Team Sky to come up with a scientifically-backed explanation for why the September 7 sample contained too much Salbutamol and the 20 other tests he gave during the race did not.
Team principal 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."
Italian rider Alessandro Petacchi was eventually given a one-year ban for taking too many puffs at the 2007 Giro. He was also stripped of the five stages he won - his Salbutamol concentration was 1,320 ng/mL.
Froome now finds himself facing a ban that could cost him one of his greatest wins, a shot at joining cycling's greats and his standing within the sport.