Pre-race favourite Alexandre Vinokourov tested positive for blood doping after winning Saturday's time-trial stage of the Tour de France.
Vinokourov was third in the 2003 Tour de France
Vinokourov has asked for a second blood sample to be tested, but his Astana team have now withdrawn from the race.
Astana said two distinct types of red blood cells were found in Vinokourov's sample indicating he had had a blood transfusion before Saturday's stage.
If Vinokourov's B-sample also tests positive he faces a two-year ban.
Leaving the Tour is a shock for the team - it's catastrophic
Astana manager Marc Biver
"It's a mistake," he told French sports newspaper L'Equipe. "I never doped, that's not the way I see my profession.
"I think it's a mistake in part due to my crash. I have spoken to the team doctors who had a hypothesis that there was an enormous amount of blood in my thighs, which could have led to my positive test.
"The setting-up of our team made a lot of people jealous and now we're paying the price. It's a shame to leave the Tour this way, but I don't want to waste time in proving my innocence."
And Astana manager Marc Biver added: "Leaving the Tour is a shock for the team. It's catastrophic. Everybody is shocked."
BLOOD DOPING EXPLAINED
What is it?
Administration of red blood cells to increase the blood's oxygen-carrying capacity
How is it done?
Injection with someone else's red blood cells; removing own blood, storing it and returning it once body has replaced it
Why do it?
The better the blood's oxygen-carrying capacity, the greater one's endurance
Blood clots, overload of circulatory system, kidney damage, transmission of infectious diseases such as HIV
Chances of being caught:
Test can only detect it if the blood comes from a donor
Vinokourov was hotly tipped for this year's title but lost time in a bad crash on stage five and slipped further back in the stages in the Alps.
But he looked back to his best in a dominant time-trial on Saturday and, although he dropped out of overall contention with a disastrous display on Sunday, he won another arduous mountain stage on Monday.
Before their withdrawal Astana had been leading the team standings in this year's Tour and had Andreas Kloeden in fifth place and Andrey Kashechkin in eighth.
The team released a statement that said: "The anti-doping control on Alexandre Vinokourov, which was carried out on July 21 after the time trial in Albi, has tested positive.
"According to the ethical code of the Astana Cycling Team Alexandre Vinokourov has been suspended from the team with immediate effect. The rider has asked nevertheless for a B-sample analysis.
"Informed by the Astana management, the organisers of the Tour de France invited the team to withdraw, which was immediately accepted."
Professional cycling has stumbled from one crisis to another in recent times.
Last year's Tour de France winner Floyd Landis is currently awaiting the verdict of a doping hearing, while Giro d'Italia winner Danilo di Luca is also being investigated.
In addition, 2006 Giro winner Ivan Basso was recently handed a two-year ban and 1996 Tour de France winner Bjarne Riis admitted using performance-enhancing drugs, as did former Telekom team-mates Eric Zabel, Udo Boelts, Bert Dietz, Christian Henn and Rolf Aldag.
Vinokourov had been an outspoken critic of current Tour leader Michael Rasmussen, who was dropped by the Danish Cycling Union team after missing two out-of-competition tests earlier in the year.
Vinokourov was third in the 2003 Tour de France and fifth in 2005 and was tipped for victory this year after a fine showing in the Dauphine Libere warm-up race.
He was also strongly favoured for last year's Tour de France but was forced to withdraw on the eve of the race when five of his team-mates were implicated in another doping scandal, leaving the team with insufficient riders to compete.