Lance Armstrong has admitted he took performance-enhancing drugs and used blood transfusions as he won his seven Tour de France titles.
In an interview with Oprah Winfrey, the 41-year-old said it was "not possible" to win the gruelling race so many times without doping.
Armstrong confessed that he doped during all seven Tour victories from 1999 to 2005, using blood-boosting agent EPO; blood doping; testosterone, cortisone or human growth hormone.
He said: "My cocktail was EPO, transfusions and testosterone. I made my decisions. They're my mistake. And I'm sitting here today to acknowledge that and to say I'm sorry for that."
The Texan, who won the world road race title in 1993, added that his drug use began in the "mid-90s".
"I suppose earlier in my career there was cortisone and then the EPO generation began," he said.
However, he said that at the time he did not believe what he was doing was cheating.
"I looked up the definition of cheat. The definition of cheat is to gain an advantage over a rival or foe," Armstrong said. "I didn't do that. I viewed it as a level playing field."
Armstrong, who has already had his Tour wins taken away and been banned from the sport for life, acknowledged his admission was probably "too late" for most people.
"I view this situation as one big lie, that I repeated a lot of times. It wasn't as if I just said no."
He had repeatedly previously denied all accusations made against him, despite the United States Anti-Doping Agency (USADA) found him to be a central figure in "the most sophisticated, professionalised and successful doping programme that sport has ever seen".
Although Armstrong acknowledged the doping, he rejected some of USADA's claims about his US Postal team, saying: "It was definitely professional and it was definitely smart, if you can call it that. But it was very conservative, very risk averse.
"But to say that that programme was bigger than the East German programme in the mid-80s, it's not true."
He denied forcing others on the team to take banned substances, but said he led by example.
Armstrong was adamant he did not dope or perform blood transfusions in 2009 or 2010, when he made his comeback to the sport, saying: "The last time I crossed that line was 2005."
He said that allegations that he did take banned substances during this comeback period were the only thing in the USADA's detailed report that "really upset" him.
It was out-of-competition testing and the so-called biological passport, which collates biological markers of doping and doping tests, that forced him to stop.
USADA chief executive Travis Tygart called the admission a "small step in the right direction".
"But if he is sincere in his desire to correct his past mistakes, he will testify under oath about the full extent of his doping activities."
The interview, which you can see again on Discovery at 8pm, was recorded in Armstrong's home town of Austin, Texas, three days before it was broadcast.
Cycling's governing body, the UCI, welcomed Armstrong's confession.
President Pat McQuaid said: "It was disturbing to watch him describe a litany of offences including, among others, doping throughout his career, leading a team that doped, bullying, consistently lying to everyone and producing a backdated medical prescription to justify a test result.
"However, Lance Armstrong also rightly said that cycling is a completely different sport today than it was 10 years ago. In particular the UCI's introduction of the biological passport in 2008 - the first sports federation to do so - has made a real difference in the fight against doping."
The International Olympic Committee (IOC), which on Thursday stripped Armstrong of his bronze medal from the 2000 Sydney Games, said: "There is a positive side if these revelations can begin to draw a line under previous practices.
"We now urge Armstrong to present all the evidence he has to the appropriate anti-doping authorities so that we can bring an end to this dark episode and move forward, stronger and cleaner."
Sky's US Correspondent Amanda Walker joined cyclists at Nello's Cycles in Armstrong's hometown of Austin, Texas, to watch the interview.
She said: "Bike enthusiasts here have followed Austin's most famous son throughout his career. Most say they knew Armstrong was doping. Finally, they got to hear it from the man himself:
"With part one of the interview broadcast, this was the verdict from some of his closest followers. Bike shop manager David Figueroa said: "I think he had a good performance. I think he spent a lot of time preparing for it and I think he's proud of himself."
"Customer and cyclist Gregg Dansom said: "As a human being, I think he's a jerk. Would I ride with him? Yeah, I don't care - I know plenty of jerks - the difference is he made up lies about people and destroyed peoples lives."
"America has another instalment to go before deciding whether to accept accept Armstrong's plea for atonement."
The Sunday Times, which was sued by Armstrong in 2004 after the paper printed allegations of his doping, said it will now be pursuing its case against him "vigorously".
A spokesman for the newspaper said: "We watched Lance Armstrong's interview with interest and noted his numerous admissions regarding taking performance-enhancing drugs.
"The Sunday Times believes that our case for recovering the £1m he obtained from us by fraud is now even stronger. We will be pursuing that case vigorously."