Lance Armstrong: US Anti-Doping Agency charges 'spiteful'
The allegations against him include previously unpublicised claims of doping in 2009 and 2010, reports say.
The 40-year-old has been immediately banned from competing in triathlons, a sport he took up after his retirement from cycling in 2011.
"I have never doped," Armstrong said in a statement on his website.
"These charges are baseless [and] motivated by spite."
In its 15-page letter, USADA said its investigation included evidence dating back to 1996 and accused Armstrong of a "pervasive pattern of doping" over many years.
It says it collected blood samples from him in 2009 and 2010 that were "fully consistent with blood manipulation including EPO use and/or blood transfusions".
More than 10 cyclists as well as cycling team employees witnessed Lance Armstrong using performance-enhancing drugs and techniques, the letter says.
He could face a lifetime ban and be stripped of his awards if he is found to have used performance-enhancing drugs, says the BBC's Jane Little in Washington.
Armstrong said on his website: "I have been notified that USADA, an organisation largely funded by taxpayer dollars but governed only by self-written rules, intends to again dredge up discredited allegations dating back more than 16 years...
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