Gordon Brown's former spin doctor has said he is "sorry and ashamed" about those affected by his actions, as he defended writing about his attacks on the ex-PM's opponents.
Damian McBride told BBC Two's Newsnight that many in the Labour Party regarded him as a "traitor" for serialising his memoirs during its annual conference.
But he added Labour must "learn lessons from his mistakes".
He said the party needed to move on from "destructive" feuds of the past.
Mr McBride has admitted leaking details about the personal lives of Labour ministers seen as a threat to Gordon Brown's ambitions to replace Tony Blair as prime minister after the 2005 election.
The revelations, published in the Daily Mail, have caused anger among senior Labour figures past and present and provided a reminder of the bitter divisions during the Blair-Brown years.
Interviewed live on Newsnight, Mr McBride said: "I do feel ashamed, I do feel sorry to those individuals whose careers I affected and even more so to the sort of innocent bystanders who were caught in the way."
In his book he suggests Mr Brown had been aware of his methods and gave his unspoken approval.
But he told Newsnight Mr Brown did not know about the "intricacies" of his actions or "understand" how he operated, and current leader Ed Miliband and shadow chancellor Ed Balls "knew even less than Gordon".
"I don't think he (Brown) knew what I was doing a lot of the time," Mr McBride said. "I operated quite a lot in the shadows."
In his first broadcast comments since the serialisation began, Mr McBride also used a recorded section with the programme to defend its timing, suggesting such books "always come out at conference season" and it would have done more damage if it had been published closer to the next election.
He added: "I know many people in the Labour movement think I'm a traitor for publishing a book lifting the lid on some of [the Blair-Brown] feuding, especially at party conference.
"But I believe if Labour's going to avoid repeating its mistakes, it's got to learn from its past, exorcise its demons, and make sure that when it says those days are over, it means it."
Interviewed later on Newsnight in front of Labour activists, Mr McBride said he agreed with Mr Balls, who has said his actions were "despicable".
Describing himself as "still a Labour supporter", Mr McBride said "there's no good time to publish a book like this".
But he added that he had been offered a "much more lucrative contract" than the £100,000-plus he received from the Daily Mail to publish the memoirs just ahead of the 2015 general election.
He said he had left his job with nothing and built up debts which the money from the book would help to pay off.
The book's revelations include claims that Mr McBride:
- held a "black book" of stories about former Home Secretary Lord Reid, who resigned from the cabinet to avoid damaging newspaper allegations
- "orchestrated a briefing war" between Charles Clarke and one of Mr Blair's advisers, leading to the ex-home secretary's sacking
- leaked a damaging story to the News of the World about then Health Minister Ivan Lewis involving a female civil servant in his private office
Mr McBride has also attempted to implicate Mr Miliband in the spin culture that prevailed in parts of the Labour government, suggesting he might "have problems" if email exchanges between Mr Miliband and spin doctor Derek Draper came to light.
Mr Miliband has insisted that although he knew negative briefings were going on, he did not approve of them and had urged Mr Brown to get rid of his aide.
Speaking on Monday, Mr Balls - a close ally of Mr Brown when he was chancellor and prime minister - said "many people", including himself, had been unaware of what Mr McBride was up to.
"What Damian McBride did was wrong and, to be honest, unconscionable," he told BBC News.
"I have never seen people behave in this way - personal, nasty, smeary, made-up stories. I think it is hideous and people are shocked by it."
Meanwhile, Conservative MP Alun Cairns has called for the police to investigate whether Mr McBride may have broken the Official Secrets Act and the Computer Misuse Act after Mr McBride admitted in his book that he had accessed Gordon Brown's email account without authorisation.
Mr McBride told Newsnight he "would not leak classified comments and I would take pains to avoid doing that... I'd be happy to talk to police if they wanted an explanation of this".
Another Tory MP, Henry Smith, demanded that Mr McBride be stripped of his civil service pension if he was found to have breached the civil service code and the special advisers' code of conduct.