|Vinokourov was third in the 2003 Tour de France
Pre-race favourite Alexandre Vinokourov tested positive for blood doping after winning Saturday's time-trial stage of the Tour de France.
Vinokourov has asked for a second blood sample to be tested, but his Astana team have now withdrawn from the race.
The Kazakh rider lost time in a bad crash on stage five and dropped out of overall contention in the Alps.
But he looked back to his best in a dominant time-trial on Saturday and won another arduous stage on Monday.
If Vinokourov's B-sample also tests positive he faces a two-year ban from cycling.
Astana said two distinct types of red blood cells were found in Vinokourov's sample indicating he had had a blood transfusion from a compatible donor shortly before Saturday's stage.
The organisers of the Tour de France invited the team to withdraw, which was immediately accepted.
Astana team statement
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 Liberty Seguros team-mates were implicated in another doping scandal, leaving the team with insufficient riders to compete.
L'Equipe said the analysis was conducted by the Chatenay-Malabry lab on the outskirts of Paris. It said two distinctive types of red blood cells were found in the A sample and showed that Vinokourov received a blood transfusion from a compatible donor shortly before the time trial.
A senior French anti-doping official confirmed to The Associated Press that there was a positive test for a blood transfusion taken from a rider at the Tour on Saturday, but said he didn't know the name of the cyclist involved. He said the test found two different types of blood, one from the rider, one from a donor.
The official spoke on condition of anonymity because no official announcement had been made.
The news spread through the Tour press corps like an electrical current.
Journalists were attending a Saunier Duval-Prodir team press conference about the team's charity program to plant 1 million trees in Mali when the first story moved across the wire: Vinokourov positive for blood doping.
David Millar was the first rider to react to the news: "Jesus Christ - there you go, that's my quote," he blurted out. "What timing, huh? This is just fucking great."
Addressing Irish sportswriter Paul Kimmage, a former cyclist who exposed doping in pro cycling his 1990 book "Rough Ride" and made waves this year when he challenged Vinokourov for working with notorious trainer Michele Ferrari at Astana's pre-race press conference, Millar said, "Well there you go, Paul."
"I wanted to believe it was a really good day [for Vinokourov]. It makes me very sad. Vino is one of my favorite riders. He's one of the most beautiful riders in the peloton. If a guy of his stature and class has done that, we all might as well pack our bags and go home right now."
Millar later backpedaled on that statement, saying, "The Tour de France should go on. If it stops, I would have to retire tomorrow."
"The irony here is that I was hoping to make an announcement today about my future plans," Millar said, likely referring to rumors that next year he will ride for Slipstream Sports, the strict anti-doping squad run by Jonathan Vaughters.
"I have some projects in the works. I am hoping to work with young riders, to show them that you don't have to dope to succeed."
Millar broke down into tears when he was asked by British journalist Jeremy Whittle if he was all right, saying, "I just feel like crying right now."