Astana team leader Alexander Vinokourov failed a dope test after winning Saturday's Tour de France time trial, his team announced.
Astana has quit the Tour de France with immediate effect following the revelation.
Astana have confirmed the positive test for blood doping.
'The anti-doping control on Alexandre Vinokourov, which was carried out on July 21 after the time trial in Albi, has tested positive,' read a statement released by the Kazakh team.
'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.'
An 'A' sample taken from the Astana rider revealed the presence of 'two distinct blood populations'.
The 33-year-old had lost all chance of winning the Tour with a dismal performance in Sunday's 14th stage but bounced back to take Monday's 15th stage in the Pyrenees where he was once again tested.
Vuelta champion Vinokourov was one of the pre-race favourites for this year's Tour but injured his knees in a crash during the first week of the race.
'I cannot comment on this until the result of the B sample's analysis,' International Cycling Union (UCI) president Pat McQuaid told Reuters news agency.
British rider David Millar, who was holding a press conference in Pau when the news broke, said: 'Jesus Christ, I'm speechless.
'It makes me sad. I have the impression the riders will never understand.'
Millar served a two-year ban and was stripped of his gold medal from the 2003 world championships after admitting taking the banned blood-booster EPO.
Elsewhere, Alessandro Petacchi has been absolved of any blame after a dope test during May's Giro d'Italia showed excessive levels of salbutamol, the Italian Cycling Federation said.
The 33-year-old sprinter was forced to miss the Tour de France after the Italian Olympic Committee (CONI), the country's anti-doping body, asked the federation to impose a one-year ban earlier this month.
However, the federation has turned down the request after Petacchi attended a hearing in Rome.
The Milram rider, who won five stages of this year's Giro, has a medical certificate to use a set amount of salbutamol for his asthma and said he should be spared a ban because any overuse was human error.
'After a 12-year career, 140 races won, I can only say that I had no reason to ruin my career with a Ventolin (inhaler). At the last Giro d'Italia I did nothing different from all the other times,' Petacchi told reporters on leaving the hearing.
Meanwhile, the team of Tour de France leader Michael Rasmussen has questioned the legality of drugs tests conducted on the rider by the Danish Anti-Doping Agency (ADD).
Dane Rasmussen is a Monaco resident and has had cycling a licence from the principality since Jan. 1, 2007. The Rabobank rider was a Mexico resident the two previous years.
'It is questionable from a legal point of view from ADD to be authorised to do tests with Michael outside Denmark because Michael has a licence from another federation,' Rabobank's lawyer Harro Knijff told a news conference.
'Until 1 January, he had a licence from the Mexican federation and as of 1 January, he has a licence from the federation of Monaco.
'It is highly questionable from a legal point of view that ADD have the competence to do tests outside Denmark,' he added.
Asked if he had ever been tested by Mexico or Monaco's authorities, Rasmussen said: 'No, I have never been tested by the Monaco authorities or by the Mexican authorities.'
Rasmussen often trains in Mexico, the home country of his wife.