"USADA is in the process of finalizing the written reasoned decision in its U.S. Postal Services pro cycling doping case," Annie Skinner said in an email to Reuters.
"We will provide the reasoned decision addressing the lifetime bans and disqualifications imposed to the UCI and WADA as provided for under the world rules. We expect it to be sent no later than October 15."
Armstrong was stripped of his record seven Tour de France wins last month and handed a lifetime ban by the USADA after indicating that he would not challenge charges that he had doped throughout his career. He has always denied doping.
USADA CEO Travis Tygart said earlier this week he expected to send the report by the end of September. No reason was given for the delay.
The International Cycling Union (UCI) is awaiting the report before confirming Armstrong's ban.
"Unless the USADA's decision and case file give serious reasons to do otherwise, the UCI has no intention to appeal to CAS (Court of Arbitration for Sport) or not to recognize the USADA's sanctions on Lance Armstrong," UCI president Pat McQuaid told Reuters in a September interview.
McQuaid said he also wanted to look into the riders who allegedly have testified against Armstrong in exchange for a reduced sentence on past doping offences.
"The UCI assumes that the decision and file will also detail the sanction the USADA may wish to enforce upon the riders who have provided testimony in exchange for reduced sanctions," he said
One of the sporting world's most polarizing figures, Armstrong remains a hero to millions of cancer survivors for beating the disease and coming back to win the Tour de France seven times. To others, he is a drug cheat and fraud.
World Anti-Doping Agency (WADA) chief John Fahey has said Armstrong's decision not to contest the allegations added up to nothing more than an admission of guilt.
"He had the right to rip up those charges, but he elected not to. Therefore the only interpretation in these circumstances is that there was substance in those charges," Fahey said.
Several former team mates including Floyd Landis and Tyler Hamilton, both admitted dopers, also have accused Armstrong of using performance-enhancing drugs.
Landis was stripped of his 2006 Tour de France title for using steroids and Hamilton, whose recent book alleged Armstrong and his U.S. Postal team mates used sophisticated techniques to avoid testing positive for the banned drugs they used, this summer lost his 2004 Olympic gold medal.
Despite the ban, Armstrong has continued to ride and run in non sanctioned races and, during a recent Montreal speech to a cancer conference, referred to himself as a seven times Tour de France winner
- Lance Armstrong
- Tour de France