Lance Armstrong faces being stripped of his seven Tour de France titles after abandoning his fight against the drug charges that have tainted his legacy.
The 40-year-old former champion announced late on Thursday that he was dropping his legal challenge against the US Anti-Doping Agency (USADA), which claim he used performance-enhancing drugs to win cycling's most prestigious race from 1999 to 2005.
Armstrong, who battled to the top of his sport after beating life-threatening cancer, maintained his innocence but said he was growing weary of the fight and the strain it had put on his personal and professional life.
"There comes a point in every man's life when he has to say 'enough is enough.' For me, that time is now," he said on his website, adding: "Finished with this nonsense."
In response, USADA chief Travis Tygart said Armstrong would be stripped of all his results dating back to August 1, 1998, adding in a separate statement that the situation was a "sad day" for sport.
The agency maintains that Armstrong used banned substances, including the blood-booster EPO and steroids, as well as blood transfusions dating back to 1996, and said 10 of his former teammates were ready to testify against him.
Armstrong argued that USADA was usurping the jurisdiction that should belong to world cycling's governing body, the International Cycling Union (UCI).
The UCI on Friday gave a guarded response, saying in a statement that it was aware of Armstrong's decision not to pursue his case.
But it said it had no comment to make other than it expected to receive a "reasoned decision" from the USADA explaining the situation.
If the UCI confirm that Armstrong will lose his Tour de France titles, they face a potential headache of choosing the new winners, as several cyclists who finished behind the American have also been implicated in doping scandals.
German rider Jan Ullrich, runner-up in 2000, 2001 and 2003 but accused earlier this year of a doping violation, refused to speculate about whether he would be handed three titles from those years.
The 1997 Tour winner said only: "I'm proud of my second-place finishes."
Another German rider who could benefit is Andreas Kloeden, who finished second in the 2004 Tour.
But his career has been hit by doping allegations, as have those of Switzerland's Alex Zulle (second in the 1999 Tour), Spain's Joseba Beloki (runner-up in 2002) and Ivan Basso of Italy (2005).
Armstrong accused USADA of an "unconstitutional witch-hunt" and received backing from the sporting director at his former US Postal and Discovery teams, Johan Bruyneel, whom the USADA have also alleged was involved in systematic doping.
"Lance has never withdrawn from a fair fight in his life so his decision today underlines what an unjust process this has been," he said on his website johanbruyneel.com
Armstrong, who retired from cycling last year, said he passed hundreds of drug tests during his career and adhered to the rules in place at the time of his Tour de France wins.
"The bottom line is I played by the rules that were put in place by the UCI, WADA (World Anti-Doping Agency) and USADA when I raced," Armstrong wrote.
"The idea that athletes can be convicted today without positive A and B samples, under the same rules and procedures that apply to athletes with positive tests, perverts the system and creates a process where any begrudged ex-teammate can open a USADA case out of spite or for personal gain or a cheating cyclist can cut a sweetheart deal for themselves."
The USADA "turned its back on its own rules, and stiff-armed those who have tried to persuade USADA to honour its obligations," he said.
"I refuse to participate in a process that is so one-sided and unfair," he added, alleging that from the start the probe had been "about punishing me at all costs."
The head of the World Anti-Doping Agency (WADA) John Fahey, however, gave his backing to the US agency.
"He (Armstrong) can say what he likes. The only way we would have known what the substance was of those charges, what the evidence was, was to have the evidence tested and I'm disappointed that won't occur," he told ABC radio in Australia.
Allegations of doping by Armstrong were made in two books, "LA Confidential" and "L.A. Official" while in 2005, the French sports daily L'Equipe reported that retested urine samples from the 1999 Tour de France indicated use of EPO.