Former US president Bill Clinton has admitted he made a "terrible moral error" by having an affair with former White House intern Monica Lewinsky.

"I think I did it for the worst possible reason -- just because I could," he said in an hour-long interview to CBS. "I think that's just about the most morally indefensible reason that anybody could have for doing anything. There are lots of sophisticated explanations, more complicated psychological explanations, but none of them are an excuse."

The interview was part of the build-up to his eagerly anticipated memoir, My Life, which is to be published Monday.

Clinton, 57, said that at the time he became involved with Lewinsky his marriage to Hillary was in trouble.

"We'd take a day a week, and we did - a whole day a week every week for a year, maybe a little more - and did counselling," said Clinton. Daughter Chelsea also was in on the therapy, he said. "We did it together. We did it individually. ... We did family work."

Clinton also said that his greatest accomplishment in office was the creation of 22 million jobs. "I kept score, how many people's lives were better off."