He is looking fitter than ever. Lighter and stronger, with his body-fat ratio surely as low as a healthy man's can go, Lance Armstrong has spent the last three weeks outdoing his own already remarkable achievements and giving cycling fans memories that should live alongside the greatest moments from the long and vivid history of the Tour de France.

What he is not receiving, however, is the unanimous admiration of those who have followed the Tour for the past three weeks.

This story of the man who beat cancer to win the world's most gruelling sporting event, which only two years ago seemed to be a feat without parallel in sport, is now being widely seen in a different light. He is at the centre of a gathering vortex of disbelief concerning his relationship with drugs.

Armstrong has consistently denied taking drugs and there is no direct evidence against him; but there were those who, right from the start, refused to believe that he could have made his way back to the top without the help of illegal methods after he had undergone treatment in 1996 for testicular cancer so advanced that it had spread to his brain and his lungs.

Two years and five months after his discharge, he did something which, in his pre-cancer years, he had failed to achieve: he won the Tour. Last year he did it again, and for much of this year's Tour he has again looked like the winner. This could only have happened, according to his accusers, with the aid of a systematic and sophisticated doping programme.

Others, formerly ready to give him the benefit of any doubt, now look at recent circumstantial evidence and feel that Armstrong has abused their trust. A year after French police seized medical refuse, including hypodermic needles and bandages, discarded by Armstrong's US Postal team during the Tour, a judicial inquiry has yet to produce its findings. And three weeks ago, as the 2001 Tour set off from Dunkirk, the Sunday Times in London published information linking Armstrong to Dr Michele Ferrari, an Italian who has worked with many cyclists and who in September will be called into a court in Bologna to answer charges of treating riders with erythropoietin, or EPO, the illegal drug that enhances endurance by increasing the proportion of red corpuscles in the blood.

The evidence against Ferrari includes the revelation by an Italian rider, Filippo Simeoni, a member of the Cantina team between 1996 and 1998, that the doctor dispensed EPO and human growth hormone to the riders and advised them on how to mask the presence of these substances.

Armstrong, hearing rumours that the Sunday Times knew the dates of his meetings with Ferrari at his clinic in Ferrara, going as far back as 1995, pre-empted the revelation by giving an interview to an Italian newspaper in which he admitted the existence of the relationship but denied that doping was a part of it.

Last week he held a press conference at which he confronted his chief accuser and again declared his innocence. "I've lived by the rules," he said. "Something like human growth hormone - you think someone with my health history would take something like that? There's no way."

Of Ferrari, he said: "I believe he's an honest man, a fair man, an innocent man. Let there be a trial. Let the man prove himself innocent."

He said he had not worked with the doctor during this Tour, but would have no qualms about resuming the relationship if Ferrari were cleared by the court.

"With what I've seen with my two eyes and my experience, how can I prosecute a man who I've never seen do anything guilty?"

Unlike many of the great cyclists of the past, he has an acute awareness of what goes on in his body. His autobiography, It's Not About the Bike, in which he described in detail the surgery and chemotherapy he endured, made it clear that he is not one of those who lie down on the operating table, close their eyes and ask no questions.

A sharp mind such as his could not be fooled into taking something illegal without knowing it. Neither would he be unaware of the precise boundaries of legality.

It would be unrealistic to believe that Armstrong went through his formative years as a junior triathlon and swimming champion in Texas without at least observing the existence of anabolic steroids. During his cancer treatment, EPO was administered. His medical knowledge, at least in a couple of specialised areas, must be well above average. And if it has enabled him to operate up to the very margin of the rules, as may be the case, then he cannot be criticised for that.

He is an extraordinary man in many ways. No one who saw him destroyed by the Alps in 1993 during his first Tour, lying in total distress in a bunk in his hostel in Serre-Chevalier three days after he had become the youngest rider ever to win a stage, could not help but have a special admiration for the determination with which he came back to master the event and eventually to dominate it.

Brash and arrogant on his arrival in big-time European cycling, he learnt his lessons fast. Although there are still those who find him difficult to like, there is no question the surly boy from a featureless Dallas suburb has adapted successfully to an alien culture. He now speaks French, Italian and Spanish well enough to conduct interviews in all three. His illness also gave him new perspectives on human existence.

"When I was sick," he said, "I saw more beauty and triumph and truth in a single day than I ever did in a bike race."

No doubt a psychologist would trace his independence and his motivation back to a difficult childhood. His mother was only 17 when he was born, and his father left before their son was two. "I've never had a single conversation with my mother about him, " Armstrong wrote. Her single-handed success in building a life for them, and her devotion to his nascent career, must also have left a powerful impression.

But there is a bigger question here than merely one man's guilt or innocence. When it comes to doping, after all, sport is always in denial of one sort or another. With cycling, history multiplies the resonances. In the old days just about everyone doped and nobody worried about it.

The first police raid on the Tour teams, at the end of a stage in Bordeaux in 1966, changed nothing. Nor, it seems, did the raids of 1998, at least if the discoveries on this year's Giro d'Italia are anything to go by.

Guilty or not, Lance Armstrong exists in a climate modified by changed attitudes and values, not least those of the mass media. Will his achievements be allowed to stand alongside those of, say, Fausto Coppi and Jacques Anquetil, or is he to be judged by a different set of criteria?

"I cannot prove a negative," he said last week, "so it's always going to be a tricky situation. When they find a test for one thing, then somebody stands up and says, 'Well, you must be doing the next thing'. When they find a test for that, then they say, 'Well, you must be using the next thing'. It goes on and on."

As well as the accusers and the recently disillusioned, there is a third group following Armstrong's progress. These are people predisposed to believe his expressions of innocence while reserving their right not to be surprised by anything the future may reveal. They are not advocates of cheating, but they acknowledge the fundamental impossibility of imposing a completely drug-free regime on bicycle racing.

Their opinion of Armstrong was summed up by a friend of mine, a German journalist covering his 19th Tour. "My admiration," he said, "is bigger than my suspicion."