World cycling has been shocked by the news that four-time Tour de France winner Chris Froome has returned an abnormal drugs test during this year's Spanish Vuelta in September.

The 32-year-old British rider was found to have exceeded the permitted levels of the asthma drug salbutamol on a test taken on September 7, cycling's world governing body, the UCI, confirmed on Wednesday.

Under World Anti-Doping Agency rules, riders are allowed a level of 1,000 nanograms per millilitre. However, Froome was found to have twice that in a urine sample taken during the Tour.

According to a UCI release 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," the statement said.

Lawyers and scientists were meanwhile working on behalf of Froome and Team Sky to challenge the result, the Guardian reported.

However, if Froome is unable to offer a sufficient explanation for the abnormal finding or challenge the result itself he will forfeit his Vuelta title under UCI rules.

Froome won the Tour de France for a fourth time in July and claimed the Vuelta title in September.

If the test result is upheld Froome could face a significant ban which may rule him out of next year's Giro D'Italia and the Tour de France, where he was planning to go for a record-equalling fifth yellow jersey.

"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," Froome said in a statement.

"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."

In a statement, Team Sky principal Sir Dave Brailsford said they were "committed to establishing the facts and understanding exactly what happened on this occasion".

"There are complex medical and physiological issues which affect the metabolism and excretion of salbutamol.

"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."

By winning the Vuelta Froome became the first cyclist since 1978 to win the gruelling Tour de France and the Spanish Tour in the same year.