The three-week test of endurance begins with a time trial around the ancient Chateau at le Prix de Veux in the Vandeaux region of France.

The organisers of the world's biggest cycling race hope that attention will be focused not on drugs, but on the sport itself.

Ahead of the start of the race, officials said compulsory blood tests to detect performance-enhancing drugs had cleared all 180 cyclists in this year's Tour de France.

Jean Lalangue of the Tour organisation: "There is a cultural revolution"

The riders from 20 teams face stringent measures aimed at eradicating the use of drugs, including taking blood samples.

The aim is to restore the reputation of professional cycling, which was badly tarnished after a series of drugs scandals and litigation.

One of the teams in last year's race was expelled following the discovery of a stock of performance-enhancing drugs, and since then a succession of drug abuse allegations has rocked cycling.

Race favourite: Richard Virenque

Exactly a year ago a support car for the Festina team was stopped by French customs officials who discovered it crammed full of performance-enhancing drugs.

The organisers of this year's event have banned two teams from this year's event and tried to exclude officials and riders who are implicated, including the famous French cyclist, Richard Virenque who is under criminal investigation in France.

But cycling's governing body, the UCI, has insisted that he is reinstated.

In addition to Virenque, the cycling union ordered Tour officials to rescind a ban on Manolo Saiz, sporting director of Spain's ONCE team.

UCI president Hein Verbruggen defends the governing body

BBC Sports correspondent Adam Mynott says this calls into question the UCI's stated commitment to rid the sport of drugs and it compromises the anti-doping efforts of the tour organisers.

Many top stars of the cycling world are not be competing in the Tour, among them defending champion Marco Pantani.

The Italian had said the course did not suit him.

But his place would have been in jeopardy anyway after he was kicked out of the Tour of Italy last month. A blood test showed he might have used a drug that increases stamina.

The two previous winners, Jan Ullrich of Germany and Bjarne Riis of Denmark, are both injured.

Striving for yellow: Chris Boardman

With the field wide open, Virenque, looks to be a favourite. Last year's surprising third-place finisher, American Bobby Julich of Cofidis, is also in contention.

British rider Chris Boardman will be hoping to pull on the first yellow jersey of this year's Tour after Saturday's prologue time trial.

Boardman's speciality is the Tour prologue - the stage is basically the world championship for the distance, and Boardman has won three.

The rider from the Wirral has finished just one Tour out of five starts, suffering bad luck with crashes apart from in 1996, when he finally reached Paris.