Ed Miliband's judgement as prime minister would be limited by his "tendency to overthink things", Damian McBride has said.
The former special adviser to Gordon Brown, whose book Power Trip is overshadowing the Labour conference in Brighton, insisted again this morning its publication would make "absolutely no difference" to the outcome of the 2015 general election campaign.
But while remaining tight-lipped on whether he had had a one-night stand with a minister during an interview on LBC, he was openly critical of Miliband's leadership qualities.
"He can have a tendency to overthink things and take too much advice," McBride said.
"I think he would benefit over time from being in the job and developing those instincts for what to do on an issue... he's got that tendency to want to hear different sides of an argument and find a balance between them."
Commentators have noted Miliband is struggling to persuade many he would be a strong prime minister - despite having more years of experience in government than previous opposition leaders Tony Blair or David Cameron.
"The longer you're a leader and you've got to make decisions, [that makes you] fall one side of the argument or the other," McBride added, suggesting Miliband would grow into the job.
He pointed out his ex-boss Brown "was at the height of his powers during the financial crisis, at the end of his career".
McBride insisted that Miliband "shows all the traits... of being someone who is such a deep thinker and has a real vision".
The serialisation of Power Trip - in a deal worth over £130,000 - has left shadow Cabinet members fuming with anger at the culture of "nefariousness" and "skulduggery" which McBride openly admits to.
Labour deputy leader Harriet Harman, once untruthfully briefed against as being "on manoeuvres", said this morning she never wants "to hear about Damian McBride again".
She told Sky News: "Lies were told against me and then they were written about in the newspapers - so the newspapers have got a lot to answer for as well.
"I'm reassured that is something in the past and we can now get on with the concerns people have in our lives."
'I loved him'
McBride himself confirmed that, in a professional sense, he loved Brown and insisted the ex-PM had never known about his underhand tactics.
"I was prepared to deal in this gutter stuff - people would come to me and tell me things they probably wouldn't tell me anything else," he explained.
"I wouldn't share those stories with Gordon or anyone, but at certain times if I thought they required it, I would share them with a journalist and it would look like their story."
McBride denied that Brown ever threw mobile phones - a consistent rumour about the PM's temper - and claimed Brown had sought to demote him rather than sack him completely because of his strong sense of personal loyalty.
He repeated his apology to all those affected by his actions and suggested he was a "functioning alcoholic" who realised he was behaving unacceptably only when those close to his politician victims began to be affected.
McBride claimed the bitter rivalry between Blair and Brown in the years leading up to Blair's departure in 2007 created the atmosphere in which his lies began to take place.
"When he [Brown] had actual enemies and critics in the Labour party that were seeking to supplant him as the natural successor to Tony Blair, that was a very brutal, bloody, difficult time," he said.
"Over time it just became one of those things I did because it was part of my relationship with journalists - to occasionally do bits of skulduggery that would get them splash headlines."