Tour de France winner Floyd Landis has tested positive for the male sex hormone testosterone, the US rider's Phonak team said on Thursday, dealing a savage blow to cycling's most prestigious race.

"The Phonak Cycling Team was notified yesterday by (world cycling body) 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 in a team statement.

It was the first time in the history of the showpiece event that its winner had given a positive drugs test during the race.

If Landis' B sample confirms the result of his A sample, the 30-year-old is certain to be stripped of his victory.

In the 17th stage, a gruelling mountain ride to Morzine in the French Alps a week ago, Landis produced an incredible comeback a day after a disastrous showing had appeared to ruin his chances of victory.

The rider from Pennsylvania crossed the line over five minutes ahead of Spaniard Carlos Sastre and went on to win the race in Paris, succeeding compatriot Lance Armstrong who retired last year after winning the Tour a record seven times.

Phonak said Landis would not ride until the matter had been clarified and said that if the B sample confirmed the positive result, the rider would be dismissed.

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

"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 statement read.

Landis pulled out of races in the last two days without giving any explanation and organisers of those events were unable to contact him.

Dutch news agency ANP quoted his team mate Koos Moerenhout as saying that Landis had pain from a hip problem and had gone to see his doctor in Germany.

The UCI said in a statement on Wednesday that a rider had tested positive during this year's race. The ruling body did not name the rider or give further details.

This year's Tour was hit by a doping scandal on the eve of the opening prologue stage.

Pre-race favourites Ivan Basso of Italy and German Jan Ullrich were forced to pull out and were suspended after being implicated in a doping investigation in Spain.

Ullrich, winner of 1997 Tour, and Basso, who was bidding for a Giro d'Italia-Tour de France double, both denied any wrong-doing.

Ullrich was later sacked by his T-Mobile sponsor while team mate Oscar Sevilla and manager Rudy Pevenage were suspended.

Nine riders, including Francisco Mancebo who was fourth in last year's race, were pulled out of the Tour because of the investigation.

The whole of the Astana-Wuerth team had to withdraw as five of their riders were on a list provided by Spanish police.

The peloton was reduced from 21 to 20 teams and from 189 to 176 riders in the biggest doping scandal since the Festina affair which rocked the 1998 Tour and brought cycling to its knees.

The investigation in Spain came to light in May when the Spanish Civil Guard raided addresses in Madrid and Zaragoza and found large quantities of anabolic steroids, equipment used for blood transfusions and more than 100 bags of frozen blood.

A notoriously tough sport, cycling has been plagued by doping for years.

Landis' win had been welcomed as thrilling by observers, who said it lifted some of the gloom hanging over the event after its traumatic start.

Asked if he had a message to deliver about doping after being crowned king of cycling, Landis had said: "In this sport, we proved that more than any sport we try to prevent doping and try to solve the problem.

However, he added: "Cycling has a reputation that doesn't seem to want to go away."

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