The disgraced Lance Armstrong maintains he did not take performance-enhancing drugs when he returned to professional cycling in 2009 and 2010.

Armstrong recently admitted to doping during each of his seven Tour de France triumphs, from 1999 to 2005. Evidence in the United States Anti-doping Agency report suggests Armstrong is not being truthful about his comeback years following his televised confession to Oprah Winfrey.

Travis Tygart, USADA's chief executive, insists there was scientific proof and on Monday Tim Herman, Armstrong's lawyer, said in emails to Press Association Sport: "I can only say that Lance is absolutely telling the truth about 2009-10."

He added: "Proving a negative is very difficult. However, the information USADA relies upon came from Lance's own website. I am not a statistical expert, but I have been told the conclusion of Mr Tygart is incorrect."

In his interview with Winfrey, Armstrong insisted he had stuck to a pact made with former wife Kristin that he would not dope during his comeback to cycling. Armstrong told Oprah: "I never would have betrayed that with her. I gave her my word and I did stick to it."

USADA revealed last year that Armstrong had led "the most sophisticated, professionalised and successful doping programme sport has ever seen". The UCI, cycling's world governing body, stripped Armstrong of his seven Tour titles - none of which were reassigned - and he was banned from sport for life.

Tygart claimed there were clear reasons for Armstrong, who was competing in triathlons, mountain bike events and marathons prior to his ban, to proclaim his innocence during those later years as it meant he would be exempt from possible criminal prosecution, because there is a five-year statute on a charge of fraud.

"The evidence is clear," Tygart told CBS' 60 Minutes programme. "His blood tests in 2009, 2010, expert reports based on the variation of his blood values from those tests, one to a million chance that it was due to something other than doping."

Eleven of Armstrong's former team-mates testified to USADA about his doping regime, in which they had been involved. Some were given bans of six months, but Tygart said there was only one course of action for Armstrong if he wanted his lifetime ban lifted.

USADA have given Armstrong a deadline of February 6 to agree to confess all under oath.