Floyd Landis, the winner of this year's Tour de France, has tested positive for unusual levels of testosterone, according to his cycling team.

The news came a day after the International Cycling Union (UCI), cycling's governing body, said that an unidentified rider had failed a drug test during the Tour. Landis had already mysteriously pulled out of two races this week.

According to his Phonak team, the American cyclist tested positive during a test after stage 17 of the race, which he won after an extraordinary climb through the Alps that was hailed as one of the great comebacks in the history of the Tour.

"The Phonak Cycling Team was notified yesterday 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," Phonak said today in a statement.

For cycling this is the worst and biggest news ever, it's the nightmare scenario, the darkest day

"The team management and the rider were both totally surprised by this physiological result."

Victory for Landis, a 30-year-old Mennonite Christian competing in his fifth tour, appeared to be out of reach after a disastrous ride in stage 16 of the Tour, in which he fell from first to 11th place, giving up eight minutes and finishing close to collapse.

But the next morning he woke up and, despite suffering extreme pain in his damaged right hip, led a 130-kilometre solo attack through the Alps to Morzine that took him back among the leaders and in prime position for his win on Saturday, in which he became the third American to win the Tour.

During a race overshadowed from the start by the suspension of 13 riders, including the two favourites, Jan Ullrich and Ivan Basso, because of their involvement in Operación Puerto, a Spanish doping investigation, Landis's recovery was scrutinised and he remained vague about how he had found the strength to come back.

"I needed to get eight minutes back, so I got angry," he told journalist after his win.

"I don’t pretend to know much about what’s going on, but my parents taught me that hard work and patience were the most important things. It took me a long time to learn that, but I think patience and persistence were key to my success."

Today Phonak said that Landis would not ride until the matter had been clarified but added that if a B sample analysis confirmed the result of the A sample, the rider would be dismissed.

"The rider will ask in the upcoming days for the counter analysis to prove either that this result has come from a natural process or that this is the result of a mistake," the team said.

Injections of testosterone, a naturally occurring steroid, help athletes develop bigger, harder muscles that recover more quickly from strenous exercise. Users of the drug are also said to enjoy the so-called "testosterone rush" of aggression that comes with a dose.

In drugs tests, testosterone levels are measured as a ratio against epitestosterone, another naturally occurring steroid with a similar chemical make-up. The most likely natural ratio is 1:1.

According to World Anti-Doping Agency regulations, a ratio of testosterone to epitestosterone that is greater than 4:1 is considered a positive result and subject to investigation.

If Landis's positive test is confirmed, he will barred from cycling for a total of four years -- two years under a Wada ban and two more under a UCI ban -- and stripped of his Tour title.

Landis was not expected to compete in next year's Tour because he is having his hip replaced, but he said after the race that he intended to take part in 2008.

Last night, Pat McQuaid, the president of the UCI, refused to name the rider who failed the test but said: "I will say that I am extremely angry and feel very let down by this. The credibility of the sport is at stake. The rider, his federation and his team have been informed of the situation."

During the race, Bradley Wiggins, the British Olympic gold medallist who finished 124th in the Tour, said there was no way of knowing how much cheating took place. "There is no leader in the peloton that everyone respects," he said. "We were getting that way with Basso, but he’s lost all the respect he had now."

Phonak, a Swiss hearing aid company, had already decide to withdraw from cycling sponsorship this year after being dogged by a series of positive drug tests among its riders. Tyler Hamilton, the Olympic time-trial champion, is serving a two-year ban for blood doping after testing positive as a Phonak rider in 2004.

"Where there’s money, there’s doping," said Andy Rihs, the company president, said recently. "But people don’t want to know about doping and that’s why the Tour will always find sponsors. Not big ones, but sponsors who have nothing to lose and everything to gain."

The 1988 Tour de France champion, Pedro Delgado, was the last winner to fail a doping test but he was never punished because probenecide, the substance found in his urine, was only on the Olympics list of banned substances.

Marco Pantani, the Italian cyclist who won the Tour in 1998 was thrown out of the 1999 Tour of Italy the day before the final stage because of a high hematrocrite level in his blood.