Chris Froome is facing questions after returning an "adverse" drugs test at the Vuelta a Espana.
The Team Sky rider had double the allowed level of legal asthma drug Salbutamol in his urine.
Cycling's world governing body the UCI wants more details from the team but says Froome is not suspended.
The Briton, 32, says he increased his dosage but it was within the legal limits and the UCI is "absolutely right" to ask questions.
Froome says he took his team doctor's advice to up his inhaler use after his asthma symptoms got worse during the Vuelta.
He became the first Briton to win the three-week race around Spain and it followed his Tour de France victory in July.
He was notified of the "adverse analytical finding" on 20 September 2017.
The urine test, taken on 7 September, showed levels of the drug, Salbutamol, which is commonly taken for asthma, were at 2,000 nanograms per millilitre (ng/ml).
That compares to the World Anti-Doping Agency's (Wada) threshold of 1,000 ng/ml.
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The use of Salbutamol is permitted, without the need of a therapeutic use exemption (TUE), but only within certain doses.
No other samples taken from Froome during the race needed further examination.
The organising body of Vuelta said it will "await the UCI's official conclusions" before any further action, adding its position is one of "extreme caution, as it hopes for this issue to be resolved as quickly as possible."
The information has only come to light following a Team Sky statement on Wednesday, issued on the back of recent media reports.
The UCI also published details of its investigation on Wednesday.
The UCI says analysis of Froome's A and B samples shows levels which exceed the limit.
Team Sky insist the rider inhaled no more than the permissible dose.
What Froome says
Froome, who has suffered with asthma since childhood, says he welcomed the investigation.
"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.
"I take my leadership position in my sport very seriously. The UCI is absolutely right to examine test results and, together with the team, I will provide whatever information it requires."
What Team Sky say
Team Sky boss Dave Brailsford said they are co-operating fully with the investigation.
"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.
"I have the utmost confidence that Chris followed the medical guidance in managing his asthma symptoms, staying within the permissible dose for Salbutamol. Of course, we will do whatever we can to help address these questions."
What the UCI says
The UCI says it is investigating Froome's case under organisation's anti-doping rules.
"The anti-doping control was planned and carried out by the Cycling Anti-Doping Foundation (CADF), the independent body mandated by the UCI, in charge of defining and implementing the anti-doping strategy in cycling.
"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.
"As a matter of principle, and whilst not required by the World Anti-Doping Code, the UCI systematically reports potential anti-doping rule violations via its website when a mandatory provisional suspension applies.
"Pursuant to Article 7.9.1. of the UCI Anti-Doping Rules, the presence of a Specified Substance such as Salbutamol in a sample does not result in the imposition of such mandatory provisional suspension against the rider."
- Four Tour de France titles
- First Briton to win Vuelta a Espana
- Two Olympic bronze medals
- Two World Championships bronze medals
Team Sky and reputation
Last week former UCI chief Brian Cookson said Team Sky should have its reputation "reinstated" following unproven doping allegations and questions over its use of therapeutic use exemptions (TUEs) - permission to use otherwise-banned substances when there is a proven medical need.
"I don't think anyone should be surprised when a professional sports team pushes the rules right to the very limit," Cookson said.
In November, UK Anti-Doping completed its investigation into allegations of wrongdoing at Team Sky and British Cycling.
The 14-month inquiry was looking into claims a 'mystery' medical package delivered for Sir Bradley Wiggins at the Criterium du Dauphine in 2011.
Ukad said it had been "unable" to prove the package contained a banned substance.
Wiggins had sought TUEs to use banned anti-inflammatory drug triamcinoclone for allergies and respiratory issues before the 2011 Tour de France, his 2012 Tour win and the 2013 Giro d'Italia.
Wiggins, British Cycling and Team Sky always denied any wrongdoing.
Froome and TUEs
Like Wiggins, Froome was named in leaked medical records by the Russian hackers Fancy Bears as one of the athletes to use TUEs during competition.
The documents claimed he was given the exemption for the asthma drug prednisolone in May 2013 and April 2014.
Earlier this year, the rider admitted he rejected a TUE for his asthma during his Tour de France win in 2015.
What now for Froome?
Analysis - Dan Roan, BBC sports editor
The greatest cyclist in the world and arguably Britain's most successful current sports star - now faces a fight to salvage his reputation. Such has been Froome's domination of his sport, and his use of medication to treat his asthma, he has repeatedly been forced to insist he is clean, and infamously faced abuse from some roadside spectators during the 2015 Tour de France. He has also been a vocal critic of "abuse" of TUEs.
Some observers have made the point that Froome made no mention or seemed to show any signs of "acute asthma symptoms" or illness during the Vuelta. Some are also surprised at Froome's announcement two weeks ago that he was riding next summer's Giro d'Italia when he privately knew that this situation could mean he may be banned for the race. But others will remain confident and hopeful that he can satisfactorily explain the elevated levels of Salbutamol, and continue his lucrative career that has earned him a £4million a year contract with Team Sky. Certainly, there will be much at stake when the UCI rules.
This could also be yet another blow to Team Sky too. Already under pressure over their use of TUEs, last month a UK Anti-Doping investigation into a mystery delivery to former rider Sir Bradley Wiggins concluded, but made clear that a lack of medical records meant that there was no evidence to back up the team's version of events.
That episode was damaging enough, but this could be much, much worse. Not just for Froome and his team, but for the whole of the sport too.
What about previous Salbutamol use in cycling?
Italian rider Diego Ulissi got a nine-month ban in 2014 for having 1920ng/ml in his test results.
His countryman Alessandro Petacchi was banned for a year for a reading of 1320ng/ml in 2007.
But riders have also been able to successfully explain adverse analytic findings. Leonardo Piepoli avoided a ban in 2007.
Should Froome not be able to similarly successfully explain the anomaly, he could be stripped of his Vuelta title and may be unable to ride in May's Giro d'Italia - as he seeks to become just the third rider to win three successive Grand Tours - or defend his Tour de France title in July.
What is Salbutamol?
According to the NHS, Salbutamol is used to relieve symptoms of asthma such as coughing, wheezing and feeling breathless.
It works by relaxing the muscles of the airways into the lungs which makes it easier to breathe.
Salbutamol comes in an inhaler, which is usually blue.
If people are unable to use an inhaler, Salbutamol can be given as tablets, capsules or syrup.
Wada introduced strict dosage regulations for 2017 for several asthma drugs - including Salbutamol - over concerns about the increase in use among athletes.
Several medical studies have suggested there is no enhancement in performance for an athlete inhaling Salbutamol.
The expert view
Dr Tom Bassindale, an anti-doping scientist at Sheffield Hallam University, now expects Froome to have laboratory tests to try to explain the abnormal result.
"The regulations allow the athlete to go through a controlled experiment where he will replicate the dosage taken and try so show why his body might have a different physiological make-up which gave the result," he told BBC Sport.
But he said he was surprised that such a common drug as Salbutamol had caused this issue.
"I wouldn't anticipate a few extra puffs on an inhaler would have any performance-enhancing effect," he added.
"The drug can have similar effects to drinking coffee - your heart beats faster, it can give you a quick boost like caffeine."
Dr Bassindale said there are a number of reasons why the test result could have been so high - but the main explanation in athletes would be dehydration.
"When the body is dehydrated, it can increase the concentration of the drug in the system," he said.
"Hours out riding a bike through the mountains might have that effect. But, having said that, Froome has been a professional athlete for some time and hasn't had any issue like this before."
Why do so many elite athletes have asthma?
Analysis - Philippa Roxby, BBC health reporter
Top athletes are more likely to have asthma than the general population.
This is down to the large volumes of air they breathe in through their mouths when exercising at high intensity over long periods of time.
When the air is cold and dry, this can trigger asthma-related symptoms such as wheezing, shortness of breath and tightness in the chest, also known as exercise-induced asthma. Cyclists are particularly at risk because of the high aerobic element of the sport. Air pollution getting into the airways out on the road can also be a trigger.
Research suggests that around 35-40% of British Olympic cyclists use an inhaler, compared with 21% of the Olympic team as a whole and 9% of the general population.
If asthma is already diagnosed in elite athletes, then intensive exercise can make it worse - but if it is properly treated, the condition should not prove a disadvantage.