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 anti-doping 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.

The finding undermines the defense that Landis has stood behind since he tested positive for an elevated ratio of testosterone to epitestosterone 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.

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’s two urine samples, the person told the Times. That test determines whether the testosterone it is natural or synthetic.

Landis officially requested the testing of his backup urine sample Monday for an elevated testosterone ratio. 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.

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