The traditional rhythm of the sporting summer - Wimbledon, the Open Championship and doping scandals on the Tour de France - gathers pace tomorrow when the 2008 Tour rolls out of Brest, in Brittany. After last year's debacle, we have been promised a fresh start, but then longstanding Tour followers have heard it all before, every year in fact since 1998, when the extent of the peloton's substance abuse was first exposed by the Festina affair.

This time, however, it might be different. “Biological passports”, a medical profile that will reveal any anomalies in the physiological make-up of the riders in the Tour peloton, were introduced at the start of this season. The objective was to ensure that riders validated their right to compete in the Tour and the race promoter, ASO, and the International Cycling Union (UCI), are cautiously optimistic that this has been achieved.

But there will be doubters and, after a decade of drugs busts, it is hard to blame people for being sceptical. Before the start of last year's Tour in London, Mark Cavendish, the British sprinter, was the first to sign a UCI ethical charter, in which the riders renounce doping. When a succession of scandals last July rendered the charter meaningless, Pat McQuaid, the UCI president, was flummoxed. “Just because they sign it doesn't really mean anything,” he said.

So the Tour's painfully slow recovery continues. Is doping attractive simply because the race is so hard? Perhaps, but then the seasoned Tour rider will admit that difficulty is measured by average speeds, not by distance or gradient. Not every day requires a lung-bursting ascent of Mont Ventoux after 200 kilometres of racing. Equally, while six or seven hours in the saddle would have most of us calling for the paramedics, professional cyclists train throughout the year with the demands of the Tour in mind. In recent years, it is clear that they have used drugs to win, rather than merely to survive.

Until last year, the Tour's popularity had been unaffected by the ethical struggle. But the first signs of collapse came when the German media, including ARD and ZDF, the broadcasters, decided to stop covering the race, appalled by the failure of the T-Mobile team to clean up their act.

After that Tour, Germany, a nation that during Jan Ullrich's heyday mustered a television audience of almost ten million for the event, washed its hands of professional racing. “There has been systematic and continued manipulation of unimaginable proportions in professional cycling,” Angela Merkel, Germany's Chancellor, said.

The draconian stance taken by ASO, and by anti-doping bodies around the world, is finally starting to bite. Only this week, Floyd Landis's attempt to overturn his conviction for doping his way to victory in the 2006 Tour was damningly dismissed by the Court of Arbitration for Sport (CAS).

Landis's defence, a familiar refrain in recent years, that he was the victim of an anti-American witch-hunt led by the French anti-doping authorities, was scoffed at by the CAS, which admonished the Pennsylvanian for his attempts to discredit the anti-doping movement, describing his defence as “unfounded”. This week, Michael Rasmussen, the Danish rider who seemed set for victory in the Tour last year until his missed doping tests caught up with him and he was sacked by Rabobank, his team, was banned for two years by the Monaco Cycling Federation, which holds his licence.

In the most radical move of all, Alberto Contador, the champion from Spain, and Astana, his sponsor, have been declared personae non gratae by the Tour organisation, after a flurry of positive tests from Astana riders last year. Contador, who has never failed a drugs test, has been connected to the Operation Puerto doping investigation in Madrid, although he has been cleared by an initial investigation.

It is almost certain that there will be doped riders in this year's peloton, playing Russian roulette with their careers. This time, however, they can expect absolutely no sympathy if their fraudulence is exposed.

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

Testing times for cyclists

2,100

Distance in miles of this year's Tour

10 million

Number of Germans who watched the 2003 Tour on television.

3 million

Number of Germans who watched the 2007 Tour on television.

12.5%

Fall in French TV ratings since 2005.

59

Percentage of Germans who believe doping is still present in cycling.

900

Number of professional cyclists submitting to biological passport tests (according to UCI figures).

23

Number of anomalies detected to date by passport procedure.

180

Number of riders on the Tour.

0

Number of past champions competing in this year's race.

460,000

Cost in euros of Astana team's anti-doping programme.

1,200

Internal tests conducted by David Millar's Garmin-Chipotle team.