Gordon Brown was "very badly served" by his aides through their "unbridled contempt" for Tony Blair, Peter Mandelson has revealed.
In an interview with The Times ahead of the publication of his political memoirs next week, the former Business Secretary said Mr Brown felt he, Lord Mandelson and Mr Blair had "killed each other" during the height of the New Labour infighting.
He also said Mr Brown "couldn't get over" the fact that Mr Blair became Labour leader and Prime Minister, and that he himself was "tripped up by Gordon and his people".
Lord Mandelson was at the heart of the New Labour project. He had to resign twice from Mr Blair's Cabinet but was sensationally brought back into Government by Mr Brown in 2008.
Lifting the lid on the corrosive relations between the men, he said Mr Brown went through three phases: pre-'94; '94 to 2007; and 2007 to 2010.
"And the middle period, as I recount in the book, was awful," he said. "That was when he kept saying to me, 'Why are we doing this to each other? We've killed each other. It's no fun. It doesn't make a minister any more enjoyable."
He added: "The unbridled contempt that some people around Gordon had for Tony and those who worked for him was very destructive. They were constantly winding him up - partly because that's what they felt. Partly because that's what they thought he wanted to hear."
Of his treatment by both former PMs, he said: "Do I wish they had perhaps behaved to me and treated me differently? Yes, I would have preferred that of course, and I would not have paid the price in my ministerial career.
"The three of us - the famous so-called Three Musketeers, as the press called us at one time - our relationship was an intense one. It was also a fantastically productive one and created New Labour after all. We also had some very good times."
His memoir, titled The Third Man, is being published more than a month ahead of Mr Blair's autobiography.