Former British prime minister Tony Blair said on Wednesday he could have not have imagined what he called the "nightmare" that unfolded in Iraq but still did not regret joining the US-led invasion.

In a political memoir Blair echoed previous statements that the 2003 invasion was justified because Saddam Hussein posed a threat and could have developed weapons of mass destruction.   

The self-penned volume "A Journey" was published on the day the United States formally ended combat operations in Iraq after a conflict that claimed more than 100,000 deaths, most of them civilians.   

Blair, 57, said he felt "desperately sorry" for the lives cut short, but said the mistaken belief that Saddam was hiding weapons of mass destruction was an "understandable error".

"I can't regret the decision to go to war ... I can say that never did I guess the nightmare that unfolded," said Blair, referring to the years of political and sectarian bloodshed in Iraq that followed the invasion.

"I have often reflected as to whether I was wrong. I ask you to reflect as to whether I may have been right."    

Blair was the closest ally of former US President George W Bush over the decision to invade Iraq. The decision was the most controversial of Blair's 10-year premiership, provoking huge protests, divisions within his Labour Party and accusations he deceived Britons over his reasons for war when weapons of mass destruction were not found.   

"I feel words of condolence and sympathy to be entirely inadequate," Blair wrote of the war's casualties. "They have died and I, the decision-maker in the circumstances that led to their deaths, still live."    

Elsewhere in the book's 715 pages Blair revealed a previously unknown reliance on alcohol to cope with the pressure of office.

Although not excessive by some standards -- a whisky before dinner followed by a half a bottle of wine -- Blair felt he was using drink as a "prop".   

And he was damning about former finance minister Gordon Brown who succeeded him as prime minister in 2007, describing his political rival as brilliant but lacking human instinct.

"Political calculation, yes. Political feelings, no. Analytical intelligence, absolutely. Emotional intelligence, zero." 

He blamed Brown for the Labour government's defeat in the May election after 13 years in power, saying it was a fatal mistake to move away from Blair's centrist "New Labour" policies.   

Blair, now an envoy for the Quartet of Middle East peacemakers -- the United States, Russia, the EU and the United Nations, was Labour's longest-serving prime minister, winning three consecutive elections before stepping down in 2007.

He has said he is donating the reported 4.6 million pound ($7.0 million) advance he received for his memoirs as well as proceeds from sales to a charity supporting serving and former members of the military. - Reuters