After more than a decade of outrunning accusations that he had doped during his celebrated cycling career, Lance Armstrong, one of the best known and most accomplished athletes in recent history, surrendered on Thursday, ending his fight against charges that he used performance-enhancing drugs.

Armstrong, who won the Tour de France an unprecedented seven straight times, said that he would not continue to contest the charges levied against him by the United States Anti-Doping Agency, which claimed that he doped and was one of the ringleaders of systematic doping on his Tour-winning teams.

He continued to deny ever doping, calling the antidoping agency’s case against him “an unconstitutional witch hunt” and saying the process it followed to deal with his matter was “one-sided and unfair.”

“There comes a point in every man’s life when he has to say, ‘Enough is enough,’ ” Armstrong said in a statement. “For me, that time is now.”

Armstrong, who turns 41 next month, said he would not contest the charges because it had taken too much of a toll on his family and his work for his cancer foundation, saying he was “finished with this nonsense.”

Armstrong’s decision, according to the World Anti-Doping Code, means he will be stripped of his seven Tour titles, the bronze medal he won at the 2000 Olympics and all other titles, awards and money he won from August 1998 forward.

It also means he will be barred for life from competing, coaching or having any official role with any Olympic sport or other sport that follows the World Anti-Doping Code. “It’s a sad day for all of us who love sport and our athletic heroes,” Travis Tygart, chief executive of the United States Anti-Doping Agency, said. “It’s yet another heartbreaking example of how the win-at-all-costs culture, if left unchecked, will overtake fair, safe and honest competition.”

As in many other high-profile doping cases — including that of the Olympic sprinter Marion Jones and other athletes involved in the sprawling Bay Area Laboratory Co-operative case, known as Balco — Tygart and the antidoping agency were basing their case not on a positive drug test but rather on other supporting evidence. Armstrong seized on that in his statement.

He said again and again that he had never tested positive — though he did test positive at the 1999 Tour for a corticosteroid, for which he produced a backdated doctor’s prescription.

Armstrong also said the case against him was flimsy without that physical evidence.

“Regardless of what Travis Tygart says, there is zero physical evidence to support his outlandish and heinous claims,” Armstrong said. “The only physical evidence here is the hundreds of controls I have passed with flying colors.”

But even without a positive test, the antidoping agency appeared set to move forward with arbitration. It claimed to have more than 10 eyewitnesses who would testify that Armstrong used banned blood transfusions, the blood booster EPO, testosterone and other drugs to win the Tour. Some of Armstrong’s closest teammates, including George Hincapie — one of the most respected American riders — were also expected to testify against him.

The antidoping agency also said it had blood test results of Armstrong’s from 2009 and 2010 that were consistent with doping.

This is not the first time a top cyclist has suffered such a career implosion — it has been common in cycling in recent years, as doping has crippled the sport. Several recent Tour de France champions have been found guilty of doping, including the American rider Floyd Landis and Alberto Contador of Spain. But none of them had the stature of Armstrong.

Although it is possible that the International Cycling Union, the world’s governing body for cycling, will appeal his suspension to the Court of Arbitration for Sport because it had battled over jurisdiction over this case, Armstrong’s choice to accept his sanction tarnishes the athletic achievements of an athlete who inspired millions with his story of cancer survival.

On Friday in Switzerland, the cycling union said in a statement that it would take no action until American officials presented their case to the association, and that it would have “no further comment” until that time.

Armstrong was already a world-champion cyclist when he was found to have testicular cancer in 1996, at 25. He overcame the odds to beat the disease. He then showed amazing strength and resilience by returning to cycling to win the Tour in 1999, gaining a mass of followers with almost a gravitational pull. They idolized him for showing that cancer could not stop him.

His legion of fans grew each year after that, and each year he won the Tour for them, turning himself into a star that transcended sports.

But in the shadows of his wild success were accusations that he had doped to win. In 1999, he tested positive for a banned corticosteroid on his way to winning his first Tour.

In 2004, the book “L.A. Confidential,” published only in French, linked Armstrong to doping, including claims by his team’s former massage therapist that he had asked her for makeup to hide needle tracks on his arm because they were evidence of his doping. In 2005, a former personal assistant claimed he found a steroid in Armstrong’s medicine cabinet.

Also in the mid-2000s, a French newspaper reported that six of Armstrong’s urine samples from the 1999 Tour had tested positive retroactively for the banned blood booster EPO. The strict standards for laboratory testing were not followed on those samples, so nothing ever came of those results.

Through the years, the accusations became more and more entangled. A Texas-based insurance company tried to withhold a $5 million performance bonus from Armstrong for his victory at the 2004 Tour because it said Armstrong had doped. Armstrong won a settlement.

In testimony in that case, Armstrong’s former teammate, Frankie Andreu, and Andreu’s wife, Betsy, said they had overheard Armstrong admitting to doctors when he was undergoing cancer treatment that he had used steroids, human growth hormone and EPO while cycling.

The accusations followed Armstrong wherever he went, but gained pace in recent years, though Armstrong’s last Tour victory continued to fade into the horizon.

Landis, who was stripped of the 2006 Tour title for doping, in 2010 accused Armstrong of doping and being involved in a doping scheme while the two were teammates. Last year, Tyler Hamilton — another Armstrong top lieutenant — told CBS that Armstrong and others on Armstrong’s teams were involved in a complex doping scheme that involved code words and secret cellphones.

Through it all, Armstrong denied doping. Even a two-year federal investigation into Armstrong that examined possible doping-related crimes seemed to come up empty. It folded earlier this year with no charges brought.

Armstrong, who retired from cycling last year, was not as fortunate this time.

He could have chosen to go to arbitration, which would have meant that witnesses could testify against him in a hearing possibly open to the public. Instead, he chose to bow out of the process.

In doing so, he emphasized that his Tour victories would always be his.

“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 said, adding: “The toughest event in the world, where the strongest man wins. Nobody can ever change that.”

He passes every single test and they bring accusations against him based on so called eyewitnesses. I want people brought down because of evidence not hearsay.

Just imagine this, he and his team was supposed to have cheated 7 straight years. Either whomever was running the tests was incompetent beyond belief or Armstrong was clean. Being the most high profile cyclist in the world you think he would have messed up at least onone of those test.

He passes every single test and they bring accusations against him based on so called eyewitnesses. I want people brought down because of evidence not hearsay.

Just imagine this, he and his team was supposed to have cheated 7 straight years. Either whomever was running the tests was incompetent beyond belief or Armstrong was clean. Being the most high profile cyclist in the world you think he would have messed up at least onone of those test.

CHEATWEAK

I am thrilled that this shameless cheater has finally been exposed - - I hope they have a high profile media event to award those seven Tour de France victories to the rightful winners.