Elite cycling team Astana have quit the Tour de France with immediate effect on Tuesday following the revelation that their lead rider Alexandre Vinokourov had failed a test for blood doping.

Vinokourov tested positive for a banned blood transfusion following his victory in Saturday's 13th stage.

The Kazakh, the pre-race favourite for the Tour, also won Monday's 15th stage between Foix and Loudenvielle and is 23rd in the overall race standings.

Vinokourov suffered a bad crash on stage five and dropped out of contention as he lost time in the Alps, but looked back to his best in a dominant time-trial on Saturday.

In an incredible turn of events, he dropped out of contention for good on Sunday after losing 28 mins 50 secs to Michael Rasmussen, the race leader, before rebounding in extraordinary fashion 24 hours later as another incredible ride in the Pyrenees gave him his second stage victory in three days.

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 had been a crowd favorite along the route this year, especially after riding through the pain barrier following his crash which left him sporting more than 60 stitches in his arms and legs.

“With a guy of his stature and class, in cycling’s current situation, we might as well pack our bags and go home,” said David Millar, the British rider who came back from a two-year doping ban in the Tour last year.

Meanwhile, Rasmussen himself expressed his regret that yet another Tour de France has been tarnished by a doping scandal.

The rider in the yellow jersey is accused of failing to inform drug testers of his whereabouts on four separate occasions. Failure to do so on three occasions constitutes a positive test under International Cycling Union (UCI) rules and should bring about a two-year suspension.

Danish Cycling Union president Jesper Worre said Rasmussen had had four warnings, two from the Danes and two from the UCI.

Rasmussen said: “I am sorry for the Tour and the organisers that this event has happened now I have the yellow jersey. Since the start I have been tested 14 times including today and I have never been positive. I want to concentrate on the race.”

Rasmussen rides for the Rabobank team, whose manager Theo de Rooy said: “There is no legal reason to stop him continuing the Tour until it ends on Sunday. When I got the warning on June 29 I took it very seriously. I contacted the UCI to announce I would fine Rasmussen.

“I selected him for Le Tour because this information was meant to be confidential. The bodies did not respect these rules of confidentiality. But I am sure many other riders in the peloton have received similar warnings.

"We have lots of international controls. We are defending a clean and responsible sport. We need a more efficient system of monitoring riders like the CSC team which serves as an example to us.”

De Rooy echoed Rasmussen’s desire for the issue not to overshadow the remainder of the race, adding: “The duel of the last few days [Rasmussen and second-placed Alberto Contador] has really thrilled people. Now we have a big mountainous stage ahead.”