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Paris - It is as much a part of French life as Bastille Day, long summer holidays and the grape harvest.

For the cyclists, it is a gruelling combination of muscle power and gritted will.

For the loyalists who follow the event, gathered on picnic blankets or perched upon their caravans, it is an obsession.

But plagued by doping scandals and mounting public scepticism, the annual Tour de France risks falling into a ditch.

A new opinion poll has found that 78 per cent of French people "sometimes or always have doubts" as to the honesty of the winner of a stage in the race.

The survey by Ifop also shows that only 52 per cent of the public are actually fond of the tour.

Among those aged over 65, nearly two-thirds say they are attached to the event.

But among those aged under 35, such feelings are expressed by only 36 per cent, reflecting a widening generational gap that worries organisers.

"We know the public doubts champions' performances. I think that's a catastrophe for the sport. Our priority, the top priority, is to restore our image and to do that we can to wage a merciless war against doping," tour organising chairman Patrice Clerc told reporters.

Doping allegations and investigations have tarnished cycling's image since 1998, when an assistant with the Festina team was discovered with a car bootful of banned substances, including the blood booster erythropoietin, or EPO. Last year the race descended into farce just days after its climax when newly crowned winner Floyd Landis tested positive for synthetic testosterone. Landis' appeal against the test result is still being processed.

So, desperate to restore credibility, organisers vowed that this year's "Grande Boucle" would be the cleanest ever. Instead, fresh scandals have forced them onto the back foot.

Just over a week into this year's tour, it was announced that the German cyclist Patrik Sinkewitz had tested positive for excessive levels of testosterone during a team training ride in early June. The news prompted the German public television networks ARD and ZDF to pull the plug on the tour, although they were swiftly replaced by two other channels.

The German sportswear manufacturer adidas and Germany's main telecommunications provider Deutsche Telekom are both reviewing their sponsorship deals with cycling. A poll appearing in the Financial Times Deutschland yesterday found that only 15 per cent of German respondents said they had a "major" or "significant" interest in this year's tour. A third said their interest in the tour had fallen this year and the overwhelming majority in this category blamed dope.

After the Sinkewitz saga erupted, tour leader Michael Rasmussen was kicked off the Danish national team by his country's cycling federation for missing two out-of-competition doping tests this year. It then emerged that Rasmussen had already been warned twice by the International Cycling Union for missing two separate random tests in the past 18 months. Undeterred, and contending that his lapses were simply "administrative errors", Rasmussen made an astonishing sprint up the mountains at the weekend that triggered dark mutterings from rival teams.

Even though the tour is mired in a battle for legitimacy as a sporting contest, TV viewership of the event is stronger than ever, leaving analysts to ponder whether this is in spite of - or because of - the negative headlines.

According to France Televisions, live coverage of the tour continued to rise last week as the Sinkewitz and Rasmussen controversy unfolded.

On average more than 3.5 million viewers watched the race each day on the channel France 2, amounting to an audience share of 40.5 per cent; at the same point in the last year's race, the figures were 3.4 million and 38 per cent respectively.