It promised to be a thrilling denouement, to decide who - Alberto Contador, Cadel Evans or Levi Leipheimer, all separated by fewer than three minutes - would wear the yellow jersey for today's final stage to Paris.

But for many, even die-hard supporters, the Tour's capacity to thrill went missing in the Pyrenees last Wednesday. Yesterday's time trial was won by Leipheimer, with Evans second and Contador fifth, and although the time gaps narrowed, it did nothing to alter the standings.

Yet even as Leipheimer threatened briefly to pull off an incredible leapfrog act there was, among many observers, a dreadful sense of anticlimax. The records will show that Contador, a Spaniard on the Lance Armstrong co-owned Discovery Channel team, will win in Paris today. At 24, he will be the youngest winner since Jan Ullrich, the 23-year-old winner ten years ago. And Evans will become the first Australian to finish on the podium in Paris, his second place bettering Phil Anderson's 5th in 1985.

It is unfortunate for Contador, Evans and Leipheimer - presuming all three are clean - that their finest achievements came at the end of the most catastrophic week in the race's history, but it is impossible not to surmise that this year's Tour will be remembered for doping rather than for racing.

Even when Contador inherited the yellow jersey, following Michael Rasmussen's expulsion on Wednesday, it seemed a case of new face, same old questions. Before joining Discovery Channel, Contador was a member of Liberty Seguros, the team that collapsed when it was heavily implicated in the Operacion Puerto investigation into blood doping. Contador was cleared of any involvement with a Madrid blood doping ring on July 26, 2006 (a year to the day before he took over the yellow jersey in this Tour) but yesterday's Le Monde newspaper alleged that some of the 200 bags of blood discovered in the clinic are labelled "AC", and suggested they belong to the young Spaniard.

"I was in a bad team, in a bad moment," said Contador when asked about the allegations last night, "and my name was linked to Operacion Puerto, but the UCI changed that. It was a mistake; I am not in this case."

When he was asked whether he would be willing to submit a DNA sample to prove that the blood is not his, he said: "I don't think it's fair, but if I have to do it, I will do it." He was asked why, if he has nothing to hide, he will not voluntarily submit to testing. "Because I'm innocent," he replied. "I don't have to prove anything. What do you want me to do? Give you my blood, my DNA...?"

This year's Tour began to unravel on Tuesday afternoon, when - on the rest day, of all days - it was announced that Alexandre Vinokourov had tested positive for blood doping. The Kazakh had suffered terrible crash injuries in the first week before bouncing back with two stunning performances, winning the stages last Saturday and Monday.

Then it was revealed that, after the first of these wins, a blood sample showed evidence of an illegal transfusion. It appeared to be a crude, old fashioned form of cheating, but Vino - though he protests his innocence - was off the race. And in an unprecedented step, his Astana team went with him, their mass withdrawal, with Andreas Kloden and Andrey Kashechkin fifth and sixth in the overall standings, interpreted by many as an admission of collective guilt.

It followed months of suspicion, which climaxed on the eve of the Tour when Astana felt compelled to issue a statement denying that they were the "men in black" - a group of riders targeted by the UCI for extra monitoring on account of their suspicious behaviour and performances. Vinokourov was also forced to defend his relationship with his trainer, the controversial Dr Michele Ferrari.

A positive test the next day for Cristian Moreni, an Italian of modest ability on the French Cofidis team, would have been a relatively minor affair had it not come 24 hours after the Vino case. Moreni was arrested as he crossed the line at the Col d'Aubisque, immediately confessed to having used synthetic testosterone, and was led to the police station.

Britain's Olympic champion Bradley Wiggins, a Cofidis team-mate of Moreni's, thus found himself out of the race. But there is no suspicion about Wiggins. He speaks unambiguously on the subject, which - strange though it may seem - tends to be enormously telling. For instance, Wiggins revealed on Friday that Moreni's behaviour had given him reason to be suspicious even before his positive test. "I had to sign a [police] form saying they could search my room and then I gave a statement," said Wiggins. "In it I mentioned my suspicions of Moreni earlier in the week, when he got angry on the team bus when the rest of us were criticising Rasmussen [when he was under fire for missing drugs tests] and saying he should be thrown off the Tour."

It was the dramatic expulsion of Rasmussen, a few hours after his win at the summit of the Aubisque, which seemed to all but guarantee him the overall victory in Paris today, that plunged the Tour to new depths. That evening, Rasmussen's team, Rabobank, pieced together one or two facts that had emerged regarding their star rider's "whereabouts". Having done so, they concluded that he had lied to them, telling them he was in Mexico in June when in fact he had been spotted in the Dolomites on June 13 and 14. Rasmussen was confronted with this information after the stage and sent home, under suspicion of lying about his whereabouts in order to avoid a drugs test.

As the Tour finishes there are some grounds for optimism - the spate of positive tests proving that the cheats aren't always a step ahead of the testers; a chasm, where one never existed before, between those who want to ride clean and those, like Moreni, who persist with their old and tainted methods. And, perhaps most significant of all, a commercial imperative to reform, given that the sport is likely to now witness an exodus of major sponsors.