"I've never committed a doping offense. I've never been involved in any act of doping," Contador said. "My promise against doping is absolute."

In a public statement read to the media, Contador said he intends to do all he can to aid investigators in the fight against doping.

"I am available to all competent authorities in the matter of doping, and will answer any questions, including providing my DNA," he said.

Flanked by Discovery Channel team boss Johann Bruyneel and Spanish Sports Minister Jaime Lissavetzky, Contador said what should have been a celebration after his Tour victory had turned sour because of the constant accusations and news coverage, even forcing him to have police protection.

"I have passed all controls ... surprise and scheduled, in my house and at competitions, during racing and when not, of blood and urine," Contador said.

"I find it impossible to understand the personal attacks against me, putting in doubt my honor as a sportsman, from people who don't know me. ... We have to make an effort to move forward in believing in cycling and in me."

The Spaniard beat Australia's Cadel Evans by 23 seconds to win in France after race leader Michael Rasmussen was removed by his team after evidence surfaced that he lied about his whereabouts during training to evade drug testing.

German authorities said last week they received information from doping expert Werner Franke which he claims show Contador was involved in doping.

Franke said he has documents from the Spanish Operation Puerto probe last year showing that Contador had taken a testosterone booster and an asthma drug. He also turned over the documents to the World Anti-Doping Agency.

But Contador had said he was mistakenly named in the Puerto investigation, which was set off last May when Spain's Civil Guard raided a Madrid clinic that allegedly provided doping services to more than 50 riders. Police seized up to 200 bags of blood, blood transfusion equipment and steroids.

Contador refused to provide DNA when questioned before a judge last year and was subsequently pulled out of the 2006 Tour along with members of his former Liberty Seguros team due to links with the Puerto case.

"It was a situation where I felt helpless, a sad and disillusioning experience that transformed the vision of the sport, the sport that I've given the best years of my life to," Contador said. "Last year, I always practiced in a clean manner, with courage, work, resignation, and a large dose of hope."

Contador said he would seek legal action against anyone making defamatory accusations in the future, and would donate any compensation won to the fight against doping.

"I don't know what else I can do," Contador said. "I, Alberto Contador, winner of the 2007 Tour, want to bring credibility ... to the new generation of cycling."