Floyd Landis, a rider with the Swiss Phonak team and the winner of this year's Tour de France, has tested positive for doping during the tour, his team has announced.
The Phonak statement came a day after cycling's governing body said an unidentified rider had failed a drug test.
Phonak said the International Cycling Union (UCI) had notified it that Landis's sample had shown "an unusual level of testosterone/epitestosterone" when he was tested after the 17th stage last Thursday.
The American rider had pulled off a remarkable feat to win the 17th stage after a disastrous 16th stage in which he fell back from first to 11th place.
The Tour de France, cycling's most prestigious event, ended on Sunday.
Phonak said Landis was suspended until the matter had been clarified. If a follow-up sample confirmed the result of the first sample, he would be dismissed.
"The team management and the rider were both totally surprised by this physiological result," the team said in a statement.
In a reaction, Landis said he hadn't cheated and was sure he could clear his name. But he said the stigma would probably remain with him.
John Lelangue, manager of Phonak, said the team would ask for the "B" sample to be analysed in the next few days.
"He will be fighting ... waiting for the B analysis and then proving to everyone that this can be natural," Lelangue said in a telephone interview.
Landis denied taking performance-enhancing drugs during the race.
"I don't know what the explanation for it [testing positive for the male sex hormone testosterone] is, whether it was a mistake or whether it's an occurrence from some other circumstances that go on in the race," he said.
Landis's win was a major success for the Phonak team, which was set up in 2000 by Andy Rihs, the chairman of the Swiss hearing aid company, and had never won the tour before.
On the eve of the Tour's start, nine riders — including pre-race favourites Jan Ullrich and Ivan Basso — were ousted, after being implicated in a Spanish doping investigation.
Lance Armstrong, who won the Tour for seven years running until 2005, retired from the sport last year.
This is not the first time Zurich-based Phonak has been hit by scandal.
In the past two years seven of the team's riders have been implicated in doping cases. Oscar Camenzind and Tyler Hamilton were among those who tested positive for banned substances.
The head of the unit that fights doping in the Swiss Federal Sport Office, Mathias Kamber, commented that the Stäfa-based Phonak team had lost even more credibility.
However, he said that even if a second test proved positive, it would be difficult for Swiss judicial authorities to take any action against the team because of the complexities.
In another reaction the director of the Tour de France, Christian Prudhomme, said race organisers would be saddened and angry if the backup test was positive.
"It is obviously distressing. This concerns the Tour's yellow jersey," he told journalists in a reference to the yellow jersey worn by the race leader and winner.
Asked if Landis risked losing his victory, Prudhomme said it was up to the UCI, which is based in the town of Aigle in western Switzerland, to decide on sanctions.
swissinfo with agencies
The 93rd Tour de France featured 20 stages.
The final stage between Sceaux-Antony and Paris was won by Norway's Thor Hushovd.
1. Floyd Landis
2. Oscar Pereiro (Spain) +59 seconds.
3. Andreas Klöden (Germany) + 1:29