Chris Froome admitted on Thursday that his adverse drugs test at the Vuelta a Espana is "damaging" as the four-time Tour de France winner battles to clear his name.

The British rider had twice the permissible amount of asthma medication Salbutamol in his system on September 7, following the 18th stage of the Grand Tour race he won for the first time.

Cycling's governing body UCI has asked the Team Sky rider to provide more information but in line with World Anti-Doping Agency guidelines has not suspended him.

That move has brought accusations of "double standards" from four-time world champion time-trialist Tony Martin, who called the decision a "scandal".

If Froome fails to provide a satisfactory answer, the UCI could proceed with an anti-doping rule violation case -- he could even be stripped of his Vuelta victory and miss a large chunk of next season.

"This is damaging. It's come as a huge shock to me as well," Froome, 32, told Sky Sports.

"At the same time, I know within me that fundamentally I have followed the protocol, I have not overstepped any boundaries and I hope by the end of this process that will be clear to everyone and I'll be exonerated of any wrongdoing.

"I can't say what other people are going to think at the end of this. I can only obviously control my input to the situation."

German Martin, though, is fuming.

"I am totally angry. There is definitely a double standard being applied in the Christopher Froome case," Martin wrote on his Facebook account.

"Other athletes are suspended immediately after a positive test. He and his team are given time by the UCI to explain it all.

"I do not know of any similar case in the recent past. That is a scandal, and he should at least not have been allowed to appear in the World Championships."

- Asthma playing up -

Froome was only informed of his adverse analytical finding on September 20, the same day he took bronze in the world championship time-trail in Bergen, Norway, when Martin finished a disappointing ninth.

Team Sky said in a statement on Wednesday that Froome experienced "acute asthma symptoms" during the final week of the Vuelta and increased his dosage of Salbutamol, within permissible limits, on medical advice.

"Coming into the last week of La Vuelta I began to feel a lot more symptomatic -- my asthma was playing up a lot more and that's when the doctor advised me to increase the number of puffs -- obviously staying well in the legal limit of the maximum allowed number of puffs you can take during the race," said Froome.

"So we did increase it and that's why we're faced with this question of 'I did stay within the limits but obviously the test results show a different reading', so we're trying to evaluate what has happened."

Like Martin, Colombian Nairo Quintana, who has twice finished second to Froome at the Tour de France but pipped him to the Vuelta title last year, said the UCI must treat everyone equally.

"We hope that the authorities do the right thing like any other rider who has gone through this situation," Quintana said at his Movistar team's presentation in Madrid on Thursday.

"The authorities should do their job well and, like any other rider, Froome has to comply with the rules."

Froome, widely considered the greatest Grand Tour rider of his generation, is scheduled to race the Giro d'Italia in May 2018 ahead of defending his Tour de France title in July 2018.

The test raises fresh questions about British cycling following the scandal surrounding Bradley Wiggins, who received therapeutic use exemptions (TUEs) to take a corticosteroid in 2011, 2012 and 2013, including before his 2012 Tour de France win.

Wiggins and Sky have repeatedly denied any wrongdoing, saying the drug was prescribed to treat a longstanding pollen allergy.