Former UK Prime Minister Tony Blair has called his successor Gordon Brown "maddening", as he outlined tensions between them in his memoirs.
He called Mr Brown a "capable and brilliant" chancellor but added he had put him under "relentless" pressure.
Mr Brown was "a strange guy" and Labour lost the recent election because it had backed away from more reforms, he said.
But Labour leadership hopeful Diane Abbott accused Mr Blair of "putting the knife" into his successor.
In his book, the former prime minister also revealed his "anguish" over the deaths of UK service personnel in Iraq but defended his decision to back the US-led invasion in 2003.
'Going on impossible
It is the first time Mr Blair has given his account of his relationship with Mr Brown while they were prime minister and chancellor - and BBC political editor Nick Robinson said it seemed things had been much worse between the two men than had been reported.
In the book, Mr Blair described Mr Brown, who succeeded him in 10 Downing Street in 2007, as a "strange guy" and said his time as prime minister was "never going to work". But it would have been "well nigh impossible" to stop him taking over, due to his power base within the party and media.
He said: "Was he difficult, at times maddening? Yes. But he was also strong, capable and brilliant, and those were qualities for which I never lost respect."
Mr Blair argued that, had he sacked or demoted Mr Brown, "the party and the government would have been severely and immediately destabilised and his ascent to the office of prime minister would probably have been even faster".
In a BBC interview to accompany the book launch, Mr Blair said his relationship with Mr Brown was "frankly hard, going on impossible" but that "for large parts of the time we were in government, he was an immense source of strength".
He added that, when Mr Brown was chancellor, "people maybe over-estimated his capacity to be prime minister" but then during "the last three years, when he was prime minister, people maybe underestimated his strengths".
'In control'
On the Iraq war, Mr Blair also insisted leaving Saddam Hussein in power would have been a "bigger risk" to security than removing him.
He acknowledged "we did not anticipate the role of al-Qaeda or Iran" in planning for the aftermath of the conflict and spoke of his "anguish" over the conflict.
"I feel desperately sorry for them, sorry for the lives cut short, sorry for the families whose bereavement is made worse by the controversy over why their loved ones died, sorry for the utterly unfair selection that the loss should be theirs."
In the book he also said he always believed he had been "in control" of his alcohol intake but admitted he had been at the "outer limit" of units of alcohol per week, something he said helped him relax and escape the pressure.
"Stiff whisky or G&T before dinner, couple of glasses of wine or even half a bottle with it. So not excessively excessive. I had a limit. But I was aware that it had become a prop."
Mr Brown's spokesman said he would not be commenting on the book, but Labour leadership candidate Diane Abbott said: "I'm surprised Tony Blair couldn't have waited a decent interval before putting the knife into Gordon Brown. It's not helpful to the party at this point."
One Labour MP close to Mr Brown told the BBC: "It is pretty much as we expected. It's certainly difficult to say there weren't tensions between them. But it is impossible to deal with the book. Once you answer one question, there are hundreds more. But we know most of it already. Tony Blair did promise to go and he chose not to. It is inevitably a one-sided account."
The former prime minister will be out of the UK on publication day, attending the opening of Middle East peace talks at the White House in Washington in his role as an envoy for the "Quartet" of the United Nations, Russia, the United States and the European Union.
