Tour de France winner Floyd Landis has given a positive drugs test, his Phonak team revealed.
Landis finished ahead of Oscar Pereiro to win the Tour de France
The 30-year-old American, who claimed Tour de France victory on Sunday, has tested positive for unusual levels of the male sex hormone testosterone.
The positive test came after stage 17 of the Tour, which saw Landis record an epic victory after struggling on the final climb the day before.
Landis has been suspended pending results of his B sample analysis.
The International Cycling Union (UCI) announced on Wednesday that a rider had failed a doping test but would not reveal his name.
The rider will not race anymore until this problem is totally clear
"The Phonak Cycling Team was notified on Wednesday by the UCI of an unusual level of testosterone/epitestosterone ratio in the test made on Floyd Landis after stage 17 of the Tour de France," said a team statement.
"The team management and the rider were both totally surprised of this physiological result.
"The rider will ask in the upcoming days for the counter analysis (B sample) to prove either that this result has come from a natural process or that this is the result of a mistake."
Landis later denied cheating and said he had high levels of testosterone occuring naturally in his body.
He also said cortisone shots he has been taking for a hip problem or medication he has been using to treat a thyroid condition might have had an effect.
Phonak said that if the second sample confirmed the positive test, Landis would be sacked.
"In application of the Pro Tour Ethical Code, the rider will not race anymore until this problem is totally clear," the statement added.
1975, October: Born Lancaster County, Pennsylvania
1990: Buys first mountain bike
1992: Junior National Mountain Bike champion
1993: US National Mountain Bike champion
1995: Moves to California
1998: Switches to road racing
1999: Joins Mercury pro team
2000: Wins Tour du Poitou-Charentes
2002: Joins US Postal, races in first Tour de France
2003, Jan: Breaks hip in crash
2003, July: Helps Armstrong win fourth Tour de France
2004: Wins Tour d'Algarve
2005: Joins Phonak, finishes ninth in Tour de France
Mar 2006: Wins Paris-Nice
Jul 2006: Wins first Tour de France
"If the result of the B sample analysis confirms the result of the A sample, the rider will be dismissed and will then pass the corresponding endocrinological examinations."
UCI spokesman Enrico Carpani said Landis had requested the analysis of the B sample.
"We are confident in the first test. For us the first one is already good," he said.
Testosterone is included as an anabolic steroid on World Anti-Doping Agency's list of banned substances and its use can be punished by a two-year ban.
Landis had already withdrawn from two races in Europe this week, Wednesday's Acht van Chaam street race in the Netherlands and Thursday's Jyske Bank Grand Prix in Denmark.
"Today, Phonak confirmed the cancellation. They could not say anything about why Landis has disappeared," said Jyske Bank Grand Prix spokesman Nils Finderup.
"We have tried to call him, to call his agent, and to call the head of the Phonak team, but no one has answered."
Organisers of the Acht van Chaam event were also angry at Landis' no-show.
Race agent John van den Akker said: "We are very annoyed. We have invested a lot of money to ensure his appearance and we would have expected some kind of explanation."
Landis finished 57 seconds ahead of Spain's Oscar Pereiro in the general classification to claim his first Tour de France title last weekend.