AUSTIN, Texas (AP) — Never one to back away from a fight, Lance Armstrong is finally giving in and the cost of quitting is steep: His seven Tour de France titles could be gone as soon as today.

The superstar cyclist, whose stirring victories after his comeback from cancer helped him transcend sports, chose not to pursue arbitration in the drug case brought against him by the U.S. Anti-Doping Agency. That was his last option in his bitter fight with USADA and his decision set the stage for the titles to be stripped and his name to be all but wiped from the record books of the sport he once ruled.

Travis Tygart, USADA's chief executive, left no doubt that was the next step. He said Armstrong would lose the titles as soon as Friday and be hit with a lifetime ban, even though he is retired and turning 41 next month.

Tygart said the UCI, the sport's governing body, was "bound to recognize our decision and impose it" as a signer of the World Anti-Doping Code.

"They have no choice but to strip the titles under the code," he said.

On Friday, the International Cycling Union said not so fast. The UCI, which had backed Armstrong's legal challenge to USADA's authority, cited the same World Anti-Doping Code in saying that it wanted the USADA to explain why Armstrong should lose his titles.

The UCI said the code requires this in cases "where no hearing occurs."

Armstrong clearly knew his legacy would be blemished by his decision. He said he has grown tired of defending himself in a seemingly never-ending fight against charges that he doped while piling up more Tour victories than anyone ever. He has consistently pointed to the hundreds of drug tests that he passed as proof of his innocence during his extraordinary run of Tour titles from 1999 to 2005.

"There comes a point in every man's life when he has to say, 'Enough is enough.' For me, that time is now," Armstrong said Thursday night, hours before the deadline to enter arbitration. He called the USADA investigation an "unconstitutional witch hunt."

"I have been dealing with claims that I cheated and had an unfair advantage in winning my seven Tours since 1999," he said. "The toll this has taken on my family and my work for our foundation and on me leads me to where I am today — finished with this nonsense."

USADA treated Armstrong's decision as an admission of guilt, hanging the label of drug cheat on an athlete who was a hero to thousands for overcoming life-threatening testicular cancer and for his foundation's support for cancer research. Armstrong could lose other awards, event titles and cash earnings, and the International Olympic Committee might look at the bronze medal he won in the 2000 Games.

"It is a sad day for all of us who love sport and athletes," Tygart said. "It's a heartbreaking example of win-at-all-costs overtaking the fair and safe option. There's no success in cheating to win."

Johan Bruyneel, Armstrong's longtime coach, said the Texan is a victim of a legal process run amok.

"Lance has never withdrawn from a fair fight in his life so his decision today underlines what an unjust process this has been," Bruyneel wrote on his personal website on Friday.

While Tygart said the agency can strip the Tour titles, Armstrong disputed that, insisting his decision is not an admission of guilt but a refusal to enter an arbitration process he believes is unfair.

"USADA cannot assert control of a professional international sport and attempt to strip my seven Tour de France titles," he said. "I know who won those seven Tours, my teammates know who won those seven Tours, and everyone I competed against knows who won those seven Tours."

Armstrong's comments notwithstanding, USADA has exercised its power to sanction athletes and strip their results regularly. Its website shows that it has issued 21 sanctions in 2012 so far in sports ranging from cycling to track to boxing to judo, with 17 of the athletes losing their results.

At the headquarters of Tour organizer ASO outside of Paris on Friday, spokesman Fabrice Tiano said Tour director Christian Prudhomme was not immediately available for comment because he was in urgent meetings about the case.

Armstrong walked away from the sport for good in 2011 without being charged following a two-year federal criminal investigation into many of the same accusations he faces from USADA.

The federal probe was closed in February, but USADA announced in June it had evidence Armstrong used banned substances and methods — and encouraged their use by teammates. The agency also said it had blood tests from 2009 and 2010 that were "fully consistent" with blood doping.

Included in USADA's evidence were emails written by Armstrong's former U.S. Postal Service teammate Floyd Landis, who was stripped of his 2006 Tour de France title after a positive drug test. Landis' emails to a USA Cycling official detailed allegations of a complex doping program on the team.

USADA also said it had 10 former Armstrong teammates ready to testify against him. Other than suggesting they include Landis and Tyler Hamilton, both of whom have admitted to doping offenses, the agency has refused to say who they are or specifically what they would say.

USADA maintains that Armstrong used banned substances as far back as 1996, including the blood-booster EPO and steroids, as well as blood transfusions.

"There is zero physical evidence to support (the) outlandish and heinous claims," Armstrong said. "The only physical evidence here is the hundreds of (doping) controls I have passed with flying colors."

Armstrong sued USADA in Austin, Texas, where he lives, in an attempt to block the case and was supported by the UCI. A judge threw out the case on Monday, siding with USADA despite questioning the agency's pursuit of Armstrong in his retirement.

"USADA's conduct raises serious questions about whether its real interest in charging Armstrong is to combat doping, or if it is acting according to less noble motives," such as politics or publicity, U.S. District Judge Sam Sparks wrote.

The ultra-competitive Armstrong still had the option to press his innocence in arbitration, which would have included a hearing during which evidence against him would have been presented. But the cyclist has said he believes most people have already made up their minds about whether he's a fraud or a persecuted hero.

And so he did something virtually unthinkable for him: He quit before a fight was over, a stunning move for an athlete who built his reputation on not only beating cancer, but forcing himself through grueling offseason workouts no one else could match, then crushing his rivals in the Alps and the Pyrenees.

"Today I turn the page. I will no longer address this issue, regardless of the circumstances," he said. "I will commit myself to the work I began before ever winning a single Tour de France title: serving people and families affected by cancer, especially those in underserved communities."

Although he had already been crowned a world champion and won individual stages at the Tour de France, Armstrong was still relatively unknown in the U.S. until he won the epic race for the first time in 1999. It was the ultimate comeback tale: When diagnosed with cancer, doctors had given him less than a 50 percent chance of survival before surgery and brutal cycles of chemotherapy saved his life.

Armstrong's riveting victories, his work for cancer awareness and his gossip-page romances with rocker Sheryl Crow, fashion designer Tory Burch and actress Kate Hudson made him a figure who transcended sports.

His dominance of the Tour de France elevated the sport's popularity in the U.S. to unprecedented levels. His story and success helped sell millions of the "Livestrong" plastic yellow wrist bracelets, and enabled him to enlist lawmakers and global policymakers to promote cancer awareness and research. His Lance Armstrong Foundation has raised nearly $500 million since its founding in 1997.

Jeffery C. Gervey, chairman of the foundation, issued a statement of support.

"Faced with a biased process whose outcome seems predetermined, Lance chose to put his family and his foundation first," Gervey said. "The leadership of the Lance Armstrong Foundation remain incredibly proud of our founder's achievements, both on and off the bike."

Questions surfaced even as Armstrong was on his way to his first Tour victory. He was leading the 1999 race when a trace amount of a banned anti-inflammatory corticosteroid was found in his urine; cycling officials said he was authorized to use a small amount of a cream to treat saddle sores.

After Armstrong's second victory in 2000, French judicial officials investigated his Postal Service team for drug use. That investigation ended with no charges, but the allegations kept coming.

Others close to Armstrong were caught up in the investigations, too: Bruyneel, the coach of Armstrong's teams, and three members of the medical staff and a consultant were also charged. Bruyneel is taking his case to arbitration, while two medical team staffers and consulting doctor Michele Ferrari didn't formally contest the charges and were issued lifetime bans by USADA. Ferrari later said he was innocent.

Armstrong was criticized for his relationship with Ferrari, who was banned by Italian authorities over doping charges in 2002. Former personal and team assistants accused Armstrong of having steroids in an apartment in Spain and disposing of syringes that were used for injections.

In 2004, a Dallas-based promotions company initially refused to pay him a $5 million bonus for winning his sixth Tour de France because it wanted to investigate allegations raised by media in Europe. Testimony in that case included former teammate Frankie Andreu and his wife, Betsy, saying Armstrong told doctors during his 1996 cancer treatments that he had taken a cornucopia of steroids and performance-enhancing drugs.

Two books published in Europe, "L.A. Confidential" and "L.A. Official," also raised doping allegations and, in 2005, French magazine L'Equipe reported that retested urine samples from the 1999 Tour showed EPO use.

Armstrong fought every accusation with denials and, in some cases, lawsuits against media outlets that reported them.

He retired in 2005 and almost immediately considered a comeback before deciding to stay on the sidelines — in part because he didn't want to keep answering doping questions. Three years later, Armstrong was 36 and itching to ride again. He came back to finish third in the 2009 Tour de France.

Armstrong raced again in 2010 under the cloud of the federal investigation. Early last year, he quit for good, making a brief return as a triathlete until the USADA investigation shut him down.

"He had a right to contest the charges," WADA President John Fahey said after Armstrong's announcement. "He chose not to. The simple fact is that his refusal to examine the evidence means the charges had substance in them."