The U.S. Anti-Doping Agency brought formal doping charges against former cyclist Lance Armstrong in an action that could cost him his seven Tour de France titles, according to a letter sent to Armstrong and several others.

As a result of the charges, Armstrong has been immediately barred from competition in Ironman triathlons, a discipline he took up after his retirement from cycling in 2011.

In the 15-page charging letter obtained by The Washington Post, USADA outlined new allegations against Armstrong, saying it collected blood samples from him in 2009 and 2010 that were “fully consistent with blood ma­nipu­la­tion including EPO use and/or blood transfusions.”

The charges represent the latest chapter in a long-running saga over whether Armstrong used banned substances during a cycling career that, along with his successful battle against testicular cancer, made him a national hero. Though for years Armstrong has successfully fended off challenges to his legacy, the action by USADA this week represents perhaps the most serious threat because of the anti-doping agency’s unique position of authority in the athletic drug-testing world.

The 12-year-old agency, which is funded jointly by the U.S. Olympic Committee and the federal government, almost never loses cases, though few athletes have the financial means or iconic status of Armstrong.

Armstrong has never tested positive for drugs, and on Wednesday he vehemently denied the USADA charges.

“I have never doped, and, unlike many of my accusers, I have competed as an endurance athlete for 25 years with no spike in performance, passed more than 500 drug tests and never failed one,” Armstrong said in a statement released by his publicist. “That USADA ignores this fundamental distinction and charges me instead of the admitted dopers says far more about USADA, its lack of fairness and this vendetta than it does about my guilt or innocence. Any fair consideration of these allegations has and will continue to vindicate me.”

In February, the U.S. Attorney’s Office in Los Angeles ended a nearly two-year investigation into doping allegations involving Armstrong without bringing criminal charges. Armstrong’s former teammates Floyd Landis and Tyler Hamilton cooperated with federal agents in that investigation and publicly accused Armstrong of doping.

USADA oversees anti-doping efforts in Olympic sports in the United States. It is empowered to bring charges that could lead to suspension from competition and the rescinding of awards. It does not have authority to bring criminal charges.

USADA’s letter, dated Tuesday, alleges that Armstrong and five former cycling team associates — three doctors including Italian physician Michele Ferrari, one trainer and team manager Johan Bruyneel — engaged in a massive doping conspiracy from 1998 to 2007, and that “the witnesses to the conduct described in this letter include more than ten (10) cyclists.”

All six, including Spanish trainer Jose “Pepi” Marti and Spanish doctors Pedro Celaya and Luis Garcia del Moral, face competition bans. USADA put all of the alleged violations in one letter, it stated, because it considers the six defendants part of a “long running doping conspiracy.”

The letter specifically alleges that “multiple riders with firsthand knowledge” will testify that Armstrong used EPO — shorthand for erythropoietin, an endurance-boosting drug — blood transfusions, testosterone and masking agents, and that he distributed and administered drugs to other cyclists from 1998 to 2005. The letter alleges that numerous witnesses will testify that Armstrong also used human growth hormone before 1996.

“These charges are a product of malice and spite and not evidence,” Robert D. Luskin, Armstrong’s Washington-based attorney, said in a telephone interview Wednesday. “Nothing else explains the fact . . . they allege an overarching doping conspiracy among four teams over 14 years and Lance is the only rider that gets charged.”

USADA chief executive Travis Tygart said in a statement that “we do not choose whether or not we do our job based on outside pressures, intimidation or for any reason other than the evidence.”

Tygart also said: “As in every USADA case, all named individuals are presumed innocent. . . . If a hearing is ultimately held then it is an independent panel of arbitrators, not USADA that determines whether or not these individuals have committed anti-doping rule violations as alleged.”

Armstrong competed for the U.S. Postal Service team and later the Discovery Channel team from 1998 to 2005. In 2009, he rode for the Astana Cycling Team and on RadioShack’s team in 2010-11.

The USADA letter further claims that Martial Saugy, the director of an anti-doping lab in Switzerland, stated that Armstrong’s urine sample results from the 2001 Tour of Switzerland indicated EPO use.

Saugy told The Post last year, however, that Armstrong’s sample was merely “suspicious,” a designation that meant it could not be called positive. Further analysis with modern methods might bring clarity, Saugy said, but the sample no longer exists.

“We did not do the additional analysis. It will never be sufficient to say, in fact, it was positive,” Saugy said in an interview. “I will never go in front of a court with that type of thing.”

Luskin said USADA sent Armstrong a letter last week asking him to meet with anti-doping officials. Armstrong declined, believing USADA was not interested in his testimony but rather a confession, Luskin said. In its letter, USADA said “with the exception of Mr. Armstrong, every other U.S. rider contacted by USADA regarding doping in cycling agreed to meet with USADA.”

Though the World Anti-Doping Agency places an eight-year statute of limitations on doping allegations, USADA argues in its letter that evidence of banned acts outside of the eight-year limit can be used to corroborate evidence within the limit, and the statute of limitations can be waived when the alleged violations were fraudulently concealed.

Armstrong, who won his last Tour de France title in 2005, has taken up competition in Ironman triathlons, and was scheduled to compete in the Ironman France in Nice on June 24.