NEW YORK -- Tests show that some of the testosterone in Floyd Landis' system at the Tour de France was synthetic and not naturally produced by his body as he claimed, according to a newspaper report.

The French antidoping lab testing the American cyclist's samples determined that some of the hormone came from an external source, The New York Times reported on its Web site Monday night, citing a person at the International Cycling Union with knowledge of the result.

A confirmation of the result would undermine the defense that Landis has stood behind since he tested positive for an abnormally high ratio of testosterone to epitestosterone in one sample taken following the 17th stage of the Tour de France, where he staged a stirring comeback in the Alps to make up for a poor performance the day before.

Looking and sounding defiant, Landis said Friday that his body's natural metabolism -- not doping of any kind -- caused the result and that he would undergo tests to prove it.

"We will explain to the world why this is not a doping case but a natural occurrence," Landis said at a news conference in Madrid, Spain.

But after determining that Landis's ratio of testosterone to epitestosterone was more than twice the limit of 4:1, the lab performed a carbon isotope ratio test on the first of Landis' two urine samples to determine whether it's natural or synthetic, the person told The Times.

Landis officially requested the testing of his backup urine sample Monday for an elevated testosterone ratio, and results were expected sometime this week. If the "B" test is negative, Landis would be cleared. If it's positive, which Landis' lawyers say they expect, he could be stripped of his Tour victory and banned for two years.

Landis has been suspended by his team, Phonak, pending the result of his "B" sample.

But the result showing synthetic testosterone does not need to be confirmed with a second test, said Dr. Gary Wadler, a member of the World Anti-Doping Agency and a spokesman for the American College of Sports Medicine.

"The rules say that it is a violation, but if you can show that the athlete had no fault or no significant fault, there could be a mitigation of the sanction," Wadler told the Times. "No matter how it got there, the athlete has to show how it got into his or her body. It could have been sabotage or contaminated dietary supplements or something else, but they have to prove how the testosterone got there."

The Times said Landis was in New York on Monday night and could not be reached for comment.

Oscar Pereiro of Spain, who finished second overall in the Tour de France, would be declared the winner if Landis loses the Tour de France title. It would be the first time in the history of the Tour of France that the winner has been disqualified for doping.

Information from The Associated Press was used in this report.