Media playback is unsupported on your device Media caption Lance Armstrong spoke about the drug allegations in February 2011

US cycling star Lance Armstrong has announced he will no longer fight drug charges from the US Anti-Doping Agency (USADA), ahead of a Friday deadline.

In a statement, the 40-year-old maintains he is innocent, but says he is weary of the "nonsense" accusations.

The USADA now says it will ban Armstrong from cycling for life and strip him of his seven Tour de France titles.

Armstrong retired from professional sport in 2011.

USADA alleges he used banned substances as far back as 1996, including the blood-booster EPO, steroid and blood transfusions.

Armstrong sued in federal court to block the charges but lost.

'Heartbreaking' case

"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 in the statement.

Lance Armstrong factfile Born: Plano, Texas

Plano, Texas Teams: Motorola, Cofidis, US Postal, Discovery Channel, Livestrong, Astana, Team RadioShack

Motorola, Cofidis, US Postal, Discovery Channel, Livestrong, Astana, Team RadioShack Tour de France: 1999, 2000, 2001, 2002, 2003, 2004, 2005 (22 individual stage wins)

1999, 2000, 2001, 2002, 2003, 2004, 2005 (22 individual stage wins) World Championships road race: 1993

1993 Battle with cancer: Diagnosed with testicular cancer in 1996. The disease spreads through his body. Launches Lance Armstrong Foundation for Cancer. Declared cancer-free in 1997 after brain surgery and chemotherapy.

Diagnosed with testicular cancer in 1996. The disease spreads through his body. Launches Lance Armstrong Foundation for Cancer. Declared cancer-free in 1997 after brain surgery and chemotherapy. Retirement: Announces he will retire after the 2005 Tour de France, which he wins. Angered by drug allegations against him, Armstrong announces in September 2008 he will return to professional cycling. In June 2010, he reveals via Twitter that the 2010 Tour de France will be his last. On 16 February 2011, Armstrong announces he will retire again.

"I have been dealing with claims that I cheated and had an unfair advantage in winning my seven Tours since 1999.

"Over the past three years, I have been subjected to a two-year federal criminal investigation followed by [USADA chief executive] Travis Tygart's unconstitutional witch hunt.

"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."

Armstrong had been given until 06:00 GMT on Friday to decide whether to continue fighting the USADA charges.

The agency has said that 10 of Armstrong's former team-mates are prepared to testify against him.

The cyclist has accused USADA of offering "corrupt inducements" to other riders.

USADA also accuses Armstrong of being a "ring-leader" of systematic doping on his Tour de France winning teams.

Media playback is unsupported on your device Media caption How does blood doping work?

Mr Tygart said shortly after Armstrong's statement that his agency would ban Armstrong from cycling for life and strip him of his titles, according to AP.

"This is a heartbreaking example of how the win-at-all-costs culture of sport, if left unchecked, will overtake fair, safe and honest competition," he said.

"But for clean athletes, it is a reassuring reminder that there is hope for future generations to compete on a level playing field without the use of performance-enhancing drugs."

However, Armstrong disputed that the USADA has the power to take away his titles.

"USADA lacks jurisdiction even to bring these charges," his statement said.

The cycling governing body the International Cycling Union (UCI) - which had backed Armstrong's challenge to challenge USADA's authority - has so far made no public comments on the latest developments.

The BBC's cycling correspondent, Simon Brotherton, said the move was unusual for Armstrong but would deny USADA the chance to directly put their evidence to him.

Though Armstrong is not admitting guilt, our correspondent adds, most people will assume that there is some kind of admission, given he is not contesting the charges when his legacy is on the line.

Armstrong, who survived testicular cancer prior to his record-breaking Tour wins, retired after the 2005 Tour de France but made a comeback in 2009.

He retired for a second time in February 2011.

He now says he will be focusing on the work with his cancer charity.