The 30-year-old American, who rides for the Swiss-based Phonak team, tested positive for the male sex hormone after an astounding comeback to win stage 17 in the French Alps, just a day after a disastrous showing which all but knocked him out of contention.
"I would like to be absolutely clear that I am not in any doping process," Landis told a news conference in Madrid in his first appearance 24 hours after news of his positive test.
"As always, since I have cycled, my physiologic parameters of testosterone and epitestosterone are high, as are those of any other sportsman. In special cases, as in mine, for natural reasons, this level is higher still," he said.
Landis will ask for a second urine sample to be tested but his lawyer Luis Sanz said he fully expected the B test to be positive -- a result which would strip him of his Tour title and result in a probable two-year ban.
It would be the first time a Tour champion has been disqualified for doping and a nightmare ending for a race that was marred before it began when nine riders were forced to pull out after being implicated in a doping investigation.
Mr Sanz said Landis would undergo hormone tests to show that his testosterone levels are naturally high and had previously hit similar levels as those tested during the Tour.
"Until the research is carried out, I ask not to be judged and much less sentenced by anybody," Landis said, his eyes welling up with tears.
"My Tour de France win was solely and exclusively the result of many years of training, of my absolute dedication to this sport, of the sacrifice of my whole life to reach my dream."
Landis started cycling at the age of 15 and was taken under the wing of seven-times Tour winner Lance Armstrong before switching to the Phonak team in 2005.
Phonak said Landis would be dismissed if the B result, which may come out on Monday, was positive. Landis said he intended to continue racing once he has had an operation on his hip.
Cycling has been blighted by a long history of doping and tour organisers said Landis's test and the pre-race doping case showed they were cracking down more heavily on transgressors.
"It is obvious that those who continue to cheat can expect a really tough time," said Tour director Christian Prudhomme.
However, the head of the World Anti-Doping Agency Dick Pound said the world cycling body UCI still had work to do to tighten their testing procedures and clean up the sport's image.
If the test is confirmed the Tour organisers will hand victory to Spain's Oscar Pereiro, who finished second overall.
Landis's lawyers could take the matter to the Court of Arbitration for Sport (CAS), a potentially long procedure.
Landis has said that after a disastrous 16th stage, when he lost the yellow jersey after falling apart on the ascent to La Toussuire, he drowned his sorrows in whiskey and beer with his teammates.
Asked if the alcohol intake could have affected his testosterone level before his remarkable stage victory a day later, the diminutive rider shrugged, smiled nervously and said: "That would be speculation but I hope not.
"I still feel satisfied I won the race and knowing how I did it. I deserved to win the race. I was the strongest guy there,” he said.
Landis said he would do everything he could to clear his name.
"I'm going to do my best to defend my dignity and my innocence," he said from Madrid, appearing on CNN’s Larry King Live program.
Landis received moral support from seven-time Tour winner Lance Armstrong, who has himself been dogged by doping allegations that he has consistently denied.
"Obviously, I was very surprised," Armstrong said of Landis's positive result.
"I was at the Tour. I watched Floyd ... I thought it was unlike anything I've seen in cycling - and I mean that in a good way,” he told Larry King on the broadcast by telephone.
But Armstrong acknowledged the affair was a blow to cycling.
"Although I still believe in Floyd and believe him to be innocent, it's not good for cycling," Armstrong said.
Armstrong said that there was never any suggestion during Landis's three years on Armstrong's US Postal team that the younger rider would be seduced by the lure of performance enhancing drugs.
"If we ever suspected anything to lead us to believe he was a cheater, we would have parted ways long before we did.
"When he did leave, he left for a better offer from another team."