Click here to view a slideshow of Lance Armstrong's past triumphs in the Tour de France

The 36-year-old won the last of his seven Tour de France titles back in 2005 before quitting the sport, but has says he will go for No 8 in the 2009 Tour in a bid to become the oldest winner in the history of the race.

"I'm going back to professional cycling," Armstrong said. "I'm going to try and win an eighth Tour de France."

Armstrong won his first Tour de France in 1999, less than three years after being diagnosed with testicular cancer. It was the first of his incredible seven consecutive victories in professional cycling's premier event.

Revealing the reasons for his comeback in a video blog on his website, the American said: "After long talks with my kids, the rest of my family, a close group of friends, I have decided to return to professional cycling in 2009. The reason for this is to launch an international cancer strategy based on the fact that we lose eight million people around the world to this disease – more than AIDS, malaria and tuberculosis combined."

Armstrong has been an outspoken advocate in the fight against cancer, raising funds and awareness of the disease. He has lobbied President Bush to increase funding for cancer research and over the weekend questioned the two US presidential candidates about how they would go about accelerating the fight against the disease.

Armstrong says that he decided to end his retirement after finishing second last month in the Leadville 100. The 100-mile mountain bike race is run through the Rocky Mountains in Colorado. "This kind of obscure bike race totally kick-started my engine," he added. "For me it's always been about the process. The process of getting there is the best part."

Armstrong, who turns 37 on 18 September, conceded that his age is starting to catch up with him but the Texas native pointed to other older athletes, including 41-year-old Olympic medallist swimmer Dara Torres, who have succeeded in their respective sports.

"I get up out of bed a little slow," he said. "I mean, I'm not going to lie. My back gets tired quicker than it used to and I get out of bed a little slower than I used to. But when I'm going, when I'm on the bike, I feel just as good as I did before.

"Older athletes are performing very well. Ask serious sports physiologists and they'll tell you age is a wives' tale.

"Athletes at 30, 35 mentally get tired. They've done their sport for 20, 25 years and they're like, 'I've had enough.' But there's no evidence to support that when you're 38 you're any slower than when you were 32."

The oldest cyclist to ever win the Tour de France was 36- year-old Firmin Lambot, who achieved the feat in 1922. The Belgian is the only cyclist over the age of 34 to win the event.

Reports of Armstrong's return began circulating on Monday when cycling website VeloNews reported that Armstrong would compete with the Astana team in five races in 2009, including the Tour de France.

Astana, managed by long-time Armstrong associate Johan Bruyneel, dismissed the suggestion but reports of the cyclists's return were fuelled when he registered with the United States Anti-Doping Agency's out-of-competition testing programme, which was a condition of his entry to the Leadville 100.

USA Cycling spokesman Andy Lee said the ruling body was "extremely excited" about the return of Armstrong.

"As a cancer survivor and seven-time Tour winner Lance has brought a tremendous amount of attention to competitive cycling in America and his success is largely responsible for the significant growth USA Cycling has experienced over the course of the last 10 years," Lee said. "A comeback to the sport's marquee event would bring with it unprecedented intrigue and excitement and would most likely have a similar impact. A return would be one of the most exciting things to happen in American cycling in a long time."