The Big Question: Is the Tour de France discredited beyond recovery by drugs scandals?

Why are we asking this now?

Because the so-called world's greatest bike race is reeling from the most damaging drugs scandal in its 104-year history. It is heading towards its conclusion in Paris on Sunday minus two entire teams, the pre-race favourite, and the rider who until his enforced withdrawal on Wednesday was wearing the leader's yellow jersey and was odds-on to emerge as the overall winner.

The integrity of the Tour has been severely compromised in the past - not least when, a few days after the end of last year's Tour, the winner was revealed to have failed a drugs test. That was bad enough, but this is much worse. The very future of the Tour is now at stake.

Wasn't this Tour supposed to herald a new, 'clean' era?

Indeed. Floyd Landis's drug-tainted 2006 victory - coming after a drugs bust in Madrid earlier that year had implicated some 50 top riders - did concentrate minds. All the more so when, a few weeks before the present Tour got under way, the man who had won in 1996 revealed that he had taken the blood-enhancer EPO.

By now teams had begun talking of zero tolerance, cycling's authorities were proclaiming that they had instituted the most rigorous drugs-testing programme of any sport, and Tour riders were required to sign a pledge of "cleanliness". The urge to make a fresh start seemed genuine, and amid the new mood the opening weekend in London and Kent was a time of huge excitement and optimism, with some three million people turning out to witness the spectacle. That all seems like a very long time ago now.

So what went wrong?

A week and a half into the Tour, a lesser German rider, Patrik Sinkewitz, was revealed to have failed a drugs test a month before the Tour began, but nothing much was made of it because he had been forced out of the race anyway through injury. Then it emerged that the race leader, the Danish rider Michael Rasmussen, had missed a series of tests in the weeks leading up to the Tour. Because he had failed no test during the Tour itself, he remained a participant, albeit with clouds of suspicion swirling round him.

The first big bombshell came three days ago when the pre-race favourite, Alexander Vinokourov of Kazakhstan, tested positive for blood-doping after he had somewhat improbably blitzed the field in a time trial, having struggled the day before. With his expulsion, and the consequent withdrawal of the rest of his Astana team, there was renewed pressure on Rasmussen, whose continued presence was proving a severe embarrassment to the Tour authorities.

On Wednesday came bombshell two: Rasmussen's Dutch Rabobank team took matters into their own hands and kicked him out. In the meantime, an Italian rider had failed a drugs test, prompting the withdrawal of his Cofidis team, which included one of the leading British riders, the Olympic champion Bradley Wiggins. The Tour was in freefall.

Why do innocents like Wiggins have to quit the Tour?

Teams are agreed that if one of their riders fails a drugs test, they will do the honourable thing and quit en masse. Such a pledge adds to the pressure on cyclists to stay clean - and it deals with the charge that no rider could cheat without some knowledge on the part of his team-mates. In this instance, Wiggins does seem very hard done by. Regarded as an an entirely honourable cyclist, he has always been outspoken in his criticisms of cycling's drug-takers. After what has happened this week, he says he wouldn't want to continue in the Tour anyway, and he is now even questioning his future in the sport.

Why do cyclists persist in cheating, knowing they'll likely be caught?

Drugs of one kind or another have been around in the Tour for as long as the Tour itself, whether in the form of amphetamines, steroids, EPO or just the injection of fresh blood (Vinokourov's offence). They are part of cycling culture. The reason: this is the toughest sports event in the world, subjecting the body to extremes of stress and pain that history suggests can't be tolerated without recourse to illegal substances.

But the fact is, it is possible to ride the Tour clean. It just might take you longer - and the lower than expected speeds on this year's Tour were taken by some to be an indication that drugs were playing a reduced role. That still leaves the question of how a rider like Vinokourov thought he could get away with it, to which most observers shake their heads in bewilderment. Vinokourov himself simply dismisses the findings - as so many cyclists in the same position before him have done.

So is cycling itself in denial?

The charge that the authorities have never really taken drug-taking seriously is a fair one, and in this they have not been helped by the cyclists themselves, who when one of their number has been fingered have tended to close ranks. But there is a very definite shift in attitude. Simply by confronting Vinokourov's offence, knowing what its effect would be, the Tour was making its own declaration. Cycling's governing body, the UCI, is also talking tougher, as it now has to, and there's a sense that the Tour has to put itself through this before it can find a way ahead. The gendarmerie has got involved this week, adding to the seriousness of the situation confronting transgressors.

But there were two developments this week of perhaps even greater significance: a protest against drug-taking by the eight Tour teams who who have formed an organisation called the Movement for Credibility in Cycling (itself an unprecedented statement from within the pro ranks), and the booing that Rasmussen was subjected to by fans when he set out on what would prove to be his final day in the saddle on Wednesday. The power of that gesture is not to be underestimated.

How are sponsors - and the French - reacting to events?

Rasmussen's team sponsors Rabobank are one of the sport's longest established, and say they will continue their support. Believing in cycling's long-term future, they point to the element of their contract that is earmarked for bringing on young riders - and by implication educating them in right and wrong. In the past, sponsors have quit the sport citing the damage to their image caused by drug-takers, but it remains to be seen how the current crop will react. Sponsors tend not to be around for more than a few years at a time anyway.

Among the French public, a poll showed that 52 per cent still loved the race despite 78 per cent saying they doubted its honesty. But France Soir ran a funeral notice for the race, and Libération argued that it should be stopped. It is refusing to publish the results. Nicolas Sarkozy, the French President, is calling for tougher anti-doping measures.

But is cancelling the Tour really an option?

No. The Tour organisers say that by continuing, they are doing right by the majority of the cyclists, which it believes are clean. Of equal concern might be the deals it has done with towns that are hosting the remaining stages. A drug-ravaged Tour is bad enough. No Tour at all would be worse.

So should the Tour be stopped?

Yes...

* The Tour has lost all credibility, the known misdeeds of a few leaving every cyclist's performance open to question

* Only by cancelling now can the Tour confront the true depth of its crisis and work out where its future lies

* It would set an example to other sports, and send a stark message to the next generation of cyclists about the perils of drug use

No...

* The transgressions of a few cyclists should not be allowed to derail an event that still represents a great sporting ideal

* The organisers have a duty to all the clean cyclists, who do not deserve to see their efforts so far come to nothing

* Fans and host towns have a huge emotional and financial investment in the continuation of the Tour