Floyd Landis, who on Sunday became the third American cyclist to win the Tour de France, tested positive for illegally high levels of testosterone during the race, his team announced today.

If a second test confirms the results, Landis may be stripped of his Tour de France championship. That has happened only once before, when Maurice Garin was disqualified in 1904 for breaking race rules. The Phonak team said that it had suspended Landis and that it would fire him if a backup test confirmed the initial result.

In a statement on its Web site, Phonak said the International Cycling Union had notified the team on Wednesday that tests on Landis’s urine sample discovered “an unusual level of testosterone/epitestosterone ratio” after Stage 17 of the Tour de France. The team and the rider were “totally surprised,” the statement said, and will ask for an analysis of a second sample taken at the same time “to prove either that this result is coming from a natural process or that this is resulting from a mistake.”

In Stage 17, Landis produced one of the greatest performances in cycling history, improbably roaring back into the lead pack after falling eight minutes behind the day before. His subsequent victory became the feel-good story that the Tour needed, particularly after the way the race had begun.