A source ‘‘familiar with the situation’’ told the Associated Press that Armstrong made the explosive revelation in a two-and-a-half-hour interview with Oprah Winfrey, taped on Monday, US time, and to be aired on Thursday.

It comes after earlier reports that Armstrong "choked up" in an apology to staff at his cancer foundation ahead of his mooted tell-all interview.

According to AP, Armstrong, 41, apologised to Livestrong foundation staff for "letting them down".

Quoting "a person with direct knowledge of the meeting", AP reported that several employees cried during Armstrong's comments, which were made during a meeting.

It was also reported that Armstrong, who did not make a direct confession about using banned drugs, urged staff to fight to continue helping cancer patients and their families.

Armstrong, a seven-time winner of the Tour de France who was stripped of the titles because of doping charges, created Livestrong in 1997 after his battle with testicular cancer. He is now battling to rebuild his reputation, according to USA Today.

The newspaper reported that in the interview with Winfrey, Armstrong would acknowledge using performance-enhancing drugs.

After the interview, Winfrey tweeted: "He came READY!".

The interview is scheduled to air on Winfrey's own television channel on Thursday, US time (Friday Australian time).

Two unidentified people with knowledge of the situation reportedly told the newspaper that Armstrong would not give any detail on specific cases and events.

In August, the US Anti-Doping Agency banned Armstrong from competition for life for "a career fuelled, start to finish, by doping". He lost his seven Tour de France titles after refusing to take the USADA case to arbitration.

The agency's report found that Armstrong forced teammates to take performance-enhancing drugs or be fired from his team, and himself transfused blood and used testosterone and erythropoietin, also known as EPO.

During a weekend jog, Armstrong told AP: "I'm calm, I'm at ease and ready to speak candidly."

He also told AP via text message that: "I told her [Winfrey] to go wherever she wants and I'll answer the questions directly, honestly and candidly. That's all I can say."

The USADA case led to a wholesale review of Cycling Australia's operations in light of admissions from two key local administrators that they had doped during their own cycling careers.