Since its inception, Team Sky has put comparable emphasis on racing clean as on winning.

In fact, clean athletes are more valuable to Team Sky than simply proven winners, so desperate are they to cultivate a reputation as a drug-free team. It has been suggested that this zero-tolerance policy is unsustainable but Sir Dave Brailsford, the team principal and mastermind behind their rise to the top, has stuck to his guns. Brailsford maintained that no one - rider or support staff - would have any association with Team Sky if they had a doping past.

That squeaky-clean reputation is being chipped away, though. The first significant blow to Team Sky came when it emerged that their former doctor, the Belgian Geert Leinders, was alleged to have been involved in doping practices during his time with the Rabobank team. Leinders' contract with Team Sky was not renewed at the end of 2012 and Brailsford has admitted subsequently that had he known the full extent of the Belgian's past, he would not have been hired.

Team Sky suffered two fresh blows this week. It was announced that Michael Rogers, the former Team Sky road captain who now rides for Saxo-Tinkoff, has been provisionally suspended after failing a drugs test for the banned substance clenbuterol.

Perhaps most damaging is the development in the Jonathan Tiernan-Locke case. Almost three months after it was leaked that he had anomalies in his biological passport values, the Union Cycliste Internationale, the world governing body, announced that the rider is to face disciplinary action over an alleged "anti-doping rule violation".

It is still not proven that Tiernan-Locke has doped, but the 28-year-old has had several months to explain the cause of unusual values in his bio-passport. He has failed to give the UCI Experts Panel a satisfactory explanation, which means that it is now left to British Cycling and UK Anti-Doping to initiate disciplinary proceedings. The case will be heard in the new year, with Tiernan-Locke denying vehemently any wrong-doing. He has, though, been suspended by Team Sky.

Irrespective of the outcome of the disciplinary hearing, Tiernan-Locke's reputation is now in the gutter. He may be proved innocent, but it seems unlikely he will not be indelibly tarnished. Cycling has too dirty a past for incidents like this to be forgiven or forgotten.

Chris Froome, winner of this year's Tour de France, has already voiced his concerns that Tiernan-Locke's case will damage his own reputation. Froome dominated the Tour this summer with no evidence that he has ever taken performance-enhancing drugs, yet he was continually bombarded with doping questions during the Tour. When asked if he feared that he would suffer by association with Tiernan-Locke, he replied: "Definitely. Inevitably. It's hugely unfortunate for the team this is now happening."

Team Sky's primary worry, though, is unlikely to be anything other than the damage which will be caused to their own reputation. Regardless of whether or not you agree with their zero-tolerance policy, one thing that Brailsford will not abide is dirty riders causing doubt that they compete for a team anything other than immaculately clean.

So keen are they to prove this, it has been reported this week that Team Sky have invited the UCI's anti-doping scientists to spend a year embedded in the team in an attempt to prove they are drug-free.

Team Sky is a juggernaut in the world of cycling. That they have been able to develop quite so quickly is because of the enormous sums of money that BSkyB pump into the team: reportedly £30m in sponsorship. The deal has been hugely beneficial to BSkyB because of the extensive exposure for the brand and the feelgood atmosphere which has been created around the team and around cycling in general.

If Team Sky lose this, they risk losing the backing of Rupert Murdoch's company. If BSkyB withdraws its sponsorship, we can all wave bye bye to Team Sky.

The team is the most professionally run in road racing, but this is only possible because of the funds at their disposal. BSkyB will not hang about a team which does not have a clean reputation. All the good work can be eroded remarkably quickly, as Brailsford will be only too well aware. He is likely to be in full damage limitation mode in coming months.

Brailsford's main cause for concern must be that Tiernan-Locke's bio-passport discrepancies occurred before he joined Team Sky. There is no suggestion of wrong-doing while he was wearing the black and blue of Team Sky, yet the rider's bio-passport was assessed by the team before he was signed and nothing unusual was discovered.

Are Team Sky doing enough due diligence? If they wish to maintain their zero-tolerance policy then their screening must become more robust to prevent another such case.

Brailsford knows that there is too much on the line for Team Sky to risk damaging their clean reputation. It will be years before the obstinate cloud of suspicion around cycling dissipates, which is to the detriment of the sport and those participating in it. Cases such as Tiernan-Locke's are certainly not helping.