Former British prime minister Tony Blair said yesterday he could not have imagined the "nightmare" that unfolded in Iraq but still did not regret joining the invasion.

In a political memoir, Blair echoed previous statements that the 2003 US-led invasion was justified because former Iraqi leader Saddam Hussein could have developed weapons of mass destruction.

The self-penned volume, A Journey, was published on the day the US formally ended combat operations in Iraq after a conflict that has claimed more than 100000 lives.

Blair said he felt "desperately sorry" for the losses of innocent lives, 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.

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