Armstrong admitted to doping during his record run of seven Tour de France wins, from 1999 to 2005, in a television interview with Oprah Winfrey last week, contradicting testimony he gave Dallas-based lawyer Jeff Tillotson in November 2005.
The long-awaited public admission opens up the 41-year-old Texan to a host of legal action.
The passage of time means criminal charges of perjury are now unlikely, but Tillotson's clients SCA Promotions is seeking to recoup 12million US dollars in prize money it paid Armstrong after losing its case more than seven years ago.
Tillotson told Press Association Sport: "Within the first minute of his interview with Oprah Winfrey he said yes to a series of questions regarding his use of performance-enhancing drugs that he had answered no to me, under oath, in giving sworn testimony.
"For us it was a very quick acknowledgement by Mr Armstrong that not only had he been lying to the public for years, but that he had lied directly and purposely to us under oath.
"The statute of limitations for Mr Armstrong to be prosecuted criminally for perjury in Texas for what he said in our case has probably run. It's probably too old a crime."
Since last Thursday and Friday night's interview, Tillotson has been in contact with Armstrong's lawyer, Austin-based Tim Herman, to inform him that SCA Promotions is prepared to open civil proceedings, if necessary.
SCA Promotions is seeking a minimum of 12m US dollars - the return of Tour de France prize money of 1.5m US dollars in 2002, 3m US dollars in 2003 and 5m US dollars in 2004, plus 2.5m in costs paid in 2004 - as well as interest and costs.
Lengthy proceedings could follow unless Armstrong agrees to a settlement.
SCA Promotions and Tillotson anticipated Armstrong, who competed in triathlons, marathons and mountain bike races before he was banned from all sport for life last year and stripped of all results from August 1, 1998, would be more cautious after years of vehement denials.
Armstrong insisted his 2005 Tour win was the last occasion he doped, despite evidence to the contrary in the United States Anti-Doping Agency report which led to his downfall.
Its reasoned decision suggested the prospect of Armstrong competing naturally during his third-placed finish in 2009 and the 2010 Tour was "less than one in a million".
USADA stood by its claims and a confession relating to his comeback years would not have impacted on the SCA Promotions case, Tillotson believes.
The lawyer described USADA's evidence as "very compelling" and believes Armstrong has an agenda to allow him to return to competitive sport.
"We have to evaluate whether this is a man truly sorry for what he did or sorry that he got caught," Tillotson added.
"Mr Armstrong made his choices, he made them deliberately, he made them forcefully and now he has to pay that price."