The Tour de France has been in rehab more times than Pete Doherty, but at Saturday's Grand Départ in Monaco, the organisers are hoping for yet another fresh start.

More than a decade after the Festina scandal and four summers after Operación Puerto, the Tour is going back to the future, with the return of Lance Armstrong seen by many as its best chance of being taken seriously again.

Yet four years on from Armstrong's valedictory speech at the 2005 Tour, the race's credibility is as fragile as ever. In that speech - with Ivan Basso and Jan Ullrich, both subsequently humbled by doping allegations, standing alongside - the American admonished the sceptics and exhorted them to “believe in these guys”.

But despite Armstrong's defence of the sport, the roll of infamy grows. Five stage victories in last year's Tour were invalidated by positive tests. One of the pre-race challengers in this year's event, Alejandro Valverde, of Spain, has been blackballed in the latest fallout from the never-ending Operación Puerto investigation.

Davide Rebellin, the Italian who is a regular winner on the European circuit, also tested positive after claiming a silver medal in the Beijing Olympic Games last year.

However, Mark Cavendish, the irrepressible British sprinter, argues that cycling is getting bad press because it has become more effective at catching cheats. “It shows that the tests are working better,” he said.

Outwardly, it appears that a lot of good work has been done. The reformed David Millar, one of the leading voices in cycling's anti-doping movement and a member of the athletes' committee of the World Anti-Doping Agency, insists that attitudes are changing.

“When I came into professional cycling, the sport had a doping culture,” Millar, 32, said. “Now it has an anti-doping culture.” His history of a fall from grace and a campaigning return to the sport after a two-year ban for taking erythropoietin (EPO), is seen as a sign of hope for many.

Yet for every David Millar, there seems to be a Bernard Kohl. The Austrian, spectacularly successful in the 2008 Tour, tested positive last October for CERA, the banned blood-boosting agent, which he, like others, had thought undetectable. Most gallingly, Kohl, third overall and King of the Mountains last July, rode for the Gerolsteiner team - as did Rebellin - whose appeal to sponsors was based on a platform of transparency.

Kohl has turned his back on cycling but his legacy is proving to be corrosive. In a series of interviews, Kohl has detailed his own doping practices, while alleging that he was far from alone. He claimed that fresh blood was flown in to last year's Tour, defrosting in the hold of the plane en route, then brought to his hotel before being reinjected at critical moments.

More damagingly, Kohl expressed cynicism over cycling's attempts to clean up its act. “The top riders are so good at doping that they know what they need to do to keep their blood levels stable to escape targeting,” he said.

Kohl's comments were supported by another confessor, Jorg Jaksche. Asked if he thought that the winner of the 2009 Tour would be using performance-enhancing drugs, the German responded: “I have experienced it myself: doping simply brings too much.”

The battle lines are more clearly defined than ever: on one side are the anti-doping campaigners, such as Millar and Greg LeMond, the former Tour winner from the United States, a clutch of journalists and a few team managers. On the other are those, such as Armstrong, who believe that, for too long, cycling has been dragged through the mud, that the continuing emphasis on doping and ethics serves only to undermine the sport further.

Armstrong, the seven-times winner of the Tour, admits that he worries about the impact of negative coverage on his children. “With news so accessible these days on the web, they'll be able to read any story they want,” Armstrong said. “And I don't want them growing up and reading all these things about me and doping.”

After years of scandal and sponsor unrest, the question is where the Tour's parent company, Amaury Sports Organisation (ASO), and the International Cycling Union (UCI) stand. With the UCI's long-awaited blood-profiling system, the “biological passport”, now in place, there appears to be far greater vigilance towards doping practices, even if there is a groundswell of informed opinion in France that says, after ten years, that ASO has grown weary of the fight.

The notion that the Tour may have thrown in the towel in the war on doping has been fuelled by the departure from the Tour's hierarchy of Patrice Clerc, its outspoken former president and a fierce critic of the UCI's laxity in the past. Clerc has consistently declined to comment since leaving ASO last autumn.

However, the French anti-doping agency (AFLD), responsible for catching several leading riders, including Floyd Landis, the 2006 “winner” who was subsequently banned for two years, will be targeting suspicious riders and will test for an as yet unspecified new drug. Samples will also be frozen during this year's Tour and tested in the future for products yet to be identified as performance-enhancing.

“We know there are some particular substances and methods, and we are going to try to detect them, sooner or later,” Pierre Bordry, head of the AFLD, said. “There are things that aren't found in blood, but I'm not going to give an example. I've learnt that the people who advise athletes on doping adopt their programmes based on drug testers' mindsets.”

Jeremy Whittle is author of Bad Blood: The Secret Life of the Tour de France (published by Random House).

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