Astana team leader Alexander Vinokourov has failed a dope test after winning Saturday's Tour de France time trial.
The team did not give any further details but it is widely believed the Kazakh, who has won the 13th and 15th stages on this year's Tour, tested positive for blood doping.
"I cannot comment on this until the result of the B sample's analysis," International Cycling Union (UCI) president Pat McQuaid said.
British rider David Millar, who was holding a press conference in Pau when the news broke, said, "I'm speechless.
"It makes me sad. I have the impression the riders will never understand."
Millar himself 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.
The controversy will cast a huge shadow over the Tour after a wretched nine months of doping scandals in cycling.
This year's race has already seen the withdrawal of T-Mobile rider Patrik Sinkewitz after he failed a pre-Tour drugs test with high levels of testosterone.
That case saw German TV stations ZDF and ARD immediately suspend their coverage of the Tour and a number of large sponsors have already stated they will review their commitment to many of the teams at the end of this year's race.
All riders in this year's Tour also signed an agreement in which they would forfeit a year's salary as well as the mandatory two year ban if they were found guilty of doping.