Some of the testosterone found in Tour de France winner Floyd Landis' "A" sample is from an external source and not his body's,
The New York Times
said on Tuesday quoting an unidentified International Cycling Union official.
The carbon isotope test on the first of Landis' two urine samples taken after his 17th stage win in last month's tour contains synthetic testosterone, said the official with knowledge of the results from France's Chatenay-Malabry antidoping laboratory.
The results, if confirmed by Landis' B sample, which the newspaper said the laboratory has agreed to test Thursday through Saturday, could make Landis the first Tour champion to be stripped of his title.
The 30-year-old American, who could also be banned from the Tour for two years, has protested his innocence to the UCI. "I'd like to make it absolutely clear that I'm not in any doping process," he said on Friday.
The UCI official said that the carbon isotope ratio test differentiates between natural and synthetic testosterone, which after Landis's surprising 17th stage win was found to be more than twice the four-to-one ratio that is allowed.
The newspaper said that Landis's personal doctor Brent Kay acknowledged that his initial test showed a ratio of 11-to-one, but cautioned that it was "not off the chart" and could be due to natural causes, bacterial contamination, alcohol consumption before the test or contamination of the specimen during testing.
Landis on Monday moved to accelerate the testing process by asking the Chatenay-Malabry laboratory to go ahead and test the B sample before it shuts for the holidays at the end of the week.
Landis has said that he expects the second sample to return a similar result to the first but insists that he is innocent.
"I ask not to be judged and much less to be sentenced by anyone," Landis said last Friday.
"I will proceed to undergo all of these tests" to show the levels "are absolutely natural and produced by my own organism", he promised on Friday.
The American added that he wished to state "categorically that my Tour win was exclusively due to many years of training and dedication" to his sport.
"I declare convincingly and categorically that my winning the Tour de France has been exclusively due to many years of training and my complete devotion to cycling.
"I was the strongest guy. I deserved to win, and I'm proud of it."
If Landis was stripped of his title he would be the first ever champion to suffer that fate.