Gordon Brown was damaged by taking controversial spin doctor Damian McBride with him from the Treasury when he became Prime Minister, according to a former Cabinet Secretary.
Lord O’Donnell, who held the post from 2005 until 2011, revealed that the then incoming Prime Minister chose to ignore advice O'Donnell had given him in 2007.
McBride was Brown's special adviser both when he was Chancellor of the Exchequer, and Prime Minister, but quit in 2009 over his involvement in a plot to smear political opponents.
Prior to his tenure serving Brown as a spin doctor, he worked for him as a Treasury Civil Servant, when Lord O’Donnell was the department’s permanent secretary.
Speaking at the Cheltenham Literature Festival, Lord O'Donnell said Mr McBride had “gone off the territory”, even as a civil servant.
"As it is clear in Damian's book, so I can talk about it now, Damian basically did not act as a civil servant," Lord O'Donnell said. “He was briefing against other ministers, as he makes clear in his book, and that's inappropriate as a civil servant.
"I had to say to Gordon Brown 'no sorry, he cannot carry on doing what he's doing and he'll have to resign'.
“So he resigns as a civil servant and gets re-appointed as a special adviser. Special advisers are appointed by ministers and I can't do anything about that.
"I advised Gordon Brown when he was coming across to No 10 not to take Damian with him and he chose to."
McBride’s memoir Power Trip was published in September, and coincided with the Labour Party’s annual conference.
However, Lord O'Donnell, who sits in the House of Lords as a crossbench peer, said that he "really liked" the idea of a "good" special adviser.
"They make the system work. Civil servants are by their nature there to be objective and not to get involved in any party issues," he said.
"Good special advisers will do those things that civil servants can't. I do think it is important though that special advisers do make a point of thinking 'my job is to help the Government'. That's the good side.
“The bad side is when they think their job is to boost their minister at the cost of other ministers. That's when it goes wrong.”