Lord Mandelson was today blamed for the "disaster" of Labour's general election campaign by one of Gordon Brown's closest allies.
In the first indication of a backlash against the former business secretary over his forthcoming memoirs, Charlie Whelan said Lord Mandelson had run the worst election campaign "in Labour's history".
Mr Whelan, political director of the Unite union, is a former press secretary to Mr Brown and remained close to him when he was prime minister.
His intervention came after Lord Mandelson, in an interview to promote his new book, said Mr Brown was "very badly served" by his aides and their "unbridled contempt" for Tony Blair.
Mr Whelan hit back, telling The Sunday Telegraph: "Peter ran the worst general election campaign in Labour's history.
"Nobody knew what the message was at all. It was a disaster from beginning to end."
He added: "Peter wasn't focused on the campaign at all. Clearly his only thoughts were for his book."
There are concerns in Labour circles about potential for the book, The Third Man, to highlight again the divisions and feuds at the top of the former Labour government.
It will be serialised from tomorrow.
Interviewed ahead of the serialisation, Lord Mandelson said that Mr Brown felt he, Lord Mandelson and Mr Blair had "killed each other" during the height of the New Labour infighting.
He suggested that Mr Brown's supporters were responsible for much of the difficulty.
"The unbridled contempt that some people around Gordon had for Tony and those who worked for him was very destructive," he said.
"They were constantly winding him up - partly because that's what they felt. Partly because that's what they thought he wanted to hear."
Lord Mandelson declined to endorse any of the current Labour leadership candidates.
But he did say he knew former Cabinet colleague Ed Miliband "very well and like him". He also said ex-Schools Secretary Ed Balls had leadership qualities, including "strong views, tough analysis and a forceful personality".
His memoir, titled The Third Man, is being published more than a month ahead of Mr Blair's long-awaited autobiography.
Lord Mandelson's timing is reported to have annoyed the former prime minister, who has held off publication of his own book until Labour was out of power.