Lance Armstrong admitted to using performance-enhancing drugs while taping a TV interview to be shown on Thursday, sources have told US media.
Last year the US Anti-Doping Agency (Usada) accused him of what it called "the most sophisticated, professionalised and successful doping program" the sport had ever seen.
Armstrong, 41, has until now publicly maintained his innocence.
He is now said to be discussing whether to testify against sport officials.
Unnamed sources familiar with Armstrong's interview with US TV personality Oprah Winfrey, which was taped on Monday, told US newspapers including the New York Times, USA Today and the Associated Press news agency that he admitted to doping during the exchange, which will be broadcast on Thursday.
But according to the New York Times source, Armstrong rebutted the claim that he was the "kingpin" of the doping programme.
The alleged confession was made just hours after Armstrong apologised to staff at the Livestrong Foundation but stopped short of a full admission of guilt.
Betsy Andreu, wife of former Armstrong teammate Frankie Andreu, was one of the first people to publicly accuse Armstrong of doping.
She told AP news of Armstrong's confession was "very emotional and very sad," and choked up when asked to comment, the news agency said.
"He used to be one of my husband's best friends and because he wouldn't go along with the doping, he got kicked to the side," she said.
"Lance could have a positive impact if he tells the truth on everything. He's got to be completely honest."
Armstrong was stripped of his seven Tour de France titles, lost most of his sponsorships and was forced to leave Livestrong after the damning Usada report.
Admitting doping might be a first step into trying to mitigate his lifetime ban from competition. He is also said to be planning to testify against powerful individuals in the world of cycling - though not other cyclists - he will claim knew about or facilitated the doping, sources said.
But his admission of guilt would raise legal issues as well as further backlash from the cycling world and cancer community.
The New York Times has reported Armstrong's supporters are concerned he could face perjury charges if he confesses to using performance-enhancing drugs, because he made sworn testimony in a 2005 court case that he had never done so.
Former teammate Floyd Landis - who was stripped of his 2006 Tour de France title for doping - has filed a federal whistle-blower lawsuit accusing Armstrong of defrauding the US Postal Service, which sponsored the team to the tune of more than $30m (£18.7m).
The US department of justice is considering whether to join the lawsuit against him, reports say, and Armstrong's lawyers are said to be in negotiations to settle the suit.
The UK's Sunday Times is already suing Armstrong for up to $1.6m over a libel payment to him in 2004 after the newspaper alleged he had cheated.
And a Texan insurance company is pursuing Lance Armstrong for $11m over insured performance bonuses paid to the American after he claimed his fourth, fifth and sixth Tour de France victories.