According to sources in France, the rider concerned is said to have tested positive for testosterone after last Thursday’s stage through the Alps to Morzine, which was won, after a 130-kilometre solo breakaway, by Floyd Landis, of the United States. Landis went on to secure overall victory in the 2006 Tour, which finished in Paris on Sunday.

Pat McQuaid, the president of the International Cycling Union (UCI) would make no comment on the rider’s identity last night. “I will say that I am extremely angry and feel very let down by this,” McQuaid said. “The credibility of the sport is at stake. The rider, his federation and his team have been informed of the situation.”

The UCI said that it would not disclose the name of the rider or other information that could reveal his identity, pending completion of the testing process.

The UCI received a report yesterday from the anti-doping laboratory at Châtenay-Malabry in Paris stating an adverse analytical finding.

“The adverse analytical finding received on Wednesday morning relates to the first analysis and will have to be confirmed either by a counter-analysis required by the rider, or by the fact that the rider renounces to that counter analysis,” the UCI statement said.

Landis was scheduled to have raced in a criterium event in the Netherlands yesterday but failed to participate, citing pain in his troublesome hip. Both he and John Lelangue, his Phonak team manager, were not available for comment last night. The Pennsylvanian, 30, is expected to have a hip replacement operation within the next two months. Halfway through the Tour, Landis gave a press conference in which he revealed that he had been suffering from intense pain from a damaged hip joint.

His race-winning breakaway came the day after he had lost eight minutes and the overall lead on the final climb to a ski station finish in La Toussuire. Speaking to the press afterwards, Landis appeared to concede defeat. “I don’t expect to win the Tour now and it’s not easy to get back eight minutes but I’ll keep fighting,” he said. “I had a very bad day on the wrong day. I suffered from the beginning. I tried to hide it, but I couldn’t. There was only one speed I could go, which wasn’t very fast.

“I never assumed the Tour was won, not at any point. I said many times that it’s possible to have a bad day.”

However, the next morning Landis went on the offensive on one of the hardest Alpine stages and rode clear of the peloton to take a spectacular solo victory in Morzine.

“I’ve heard it said that I had a great comeback but I’ll let other people be the judge of that,” he said. “I’m proud of the way my team raced and pleased with how I rode.”

Landis rides for the Phonak team, who have been dogged by controversy after a series of positive tests by their best-known riders. Tyler Hamilton, the Olympic time-trial champion, is serving a two-year ban for blood doping, imposed when he was a Phonak rider.

Phonak are withdrawing from sponsorship of cycling at the end of this season. Despite Landis’s Tour win, the continuing scandals in cycling have become unpalatable for Andy Rihs, Phonak’s president.

“Where there’s money, there’s doping,” Rihs said. “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.”

Phonak will be replaced at the end of this season by Ishares, a subsidiary of Barclays Bank. “I told them, ‘We have left you an ethical code within this team that’s more strict than that of the UCI,’ ” Rihs said. “ ‘Think hard before you get involved in cycling, because there are never any guarantees when it comes to doping.’ ”

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