Tony Blair was so depressed after the Iraq war that the then-British prime minister told Gordon Brown he would quit the following summer, only to renege on his pledge, a British newspaper report says.

The physical and mental stress on Mr Blair was so profound that he confided to friends he "spaced out" several times during his weekly session of parliamentary questions, according to a new book serialised in The Observer.

The End of the Party, by political journalist Andrew Rawnsley, claims Mr Blair was haunted by the chaos and bloodshed in Iraq, coupled with the constant pressure from Mr Brown, the-then finance minister, to step aside.

Mr Rawnsley claims Mr Blair was able to hide his depression from the public and most of his staff, but privately he decided to bow to Mr Brown's wishes and hand over to him midway through his second term.

But Mr Blair, who was former US president George W Bush's staunchest ally in the 2003 US-led invasion of Iraq, went on to lead his Labour Party to victory in the 2005 general election and only stood down as prime minister two years later.

The book says Mr Blair, in his darkest days, made clear at a dinner with both Mr Brown and Deputy Prime Minister John Prescott in November 2003, as well as later in a telephone call to Mr Prescott in spring 2004, that he would step aside.

Sally Morgan, Mr Blair's director of government relations, told Mr Rawnsley: "Iraq was a quicksand swallowing him up. The atrocities. Those terrible photos [of abuse of prisoners at the US-run Abu Ghraib jail]."

"And he started losing people who had supported him throughout. He was stuck in this long dark tunnel and could see no way out of it."

The book relates how Mr Blair's special envoy in Iraq, the former UN ambassador Jeremy Greenstock, briefed Mr Blair at the end of his service in Baghdad.

Mr Rawnsley says Mr Greenstock knew that his "very gloomy assessment" had made him highly unpopular in Mr Blair's offices and some officials had tried to keep him away, fearing the impact on the prime minister's already depressed morale.

The book claims that when Mr Greenstock warned Mr Blair that the situation looked "unbelievably bad" and would become more desperate in the months to come, Mr Blair pleaded: "What can we do?"

He is also said to have asked: "We have told them [the Americans] again and again what we think is necessary. If it doesn't happen, what can we do?"

Mr Greenstock was left with the image of the prime minister "tearing his hair" over Iraq and "throwing his hands in the air", Mr Rawnsley writes.

But Mr Blair then apparently regained his self belief and, encouraged by his wife Cherie, decided to fight on, to the immense anger of Mr Brown.

Rawnsley claims Mr Brown went to Mr Blair's Downing Street office to confront his rival.

One of Mr Blair's inner circle who witnessed the showdown told Mr Rawnsley: "Gordon was just losing it. He was behaving like a belligerent teenager. Just standing in the office shouting: 'When are you going to f***ing go?'"

In last week's excerpts from the book, Mr Rawnsley claimed that once Mr Brown became prime minister he had to be warned by the top civil servant about his intimidating behaviour towards staff.

- AFP