Addressing his Labour Party’s annual conference, Blair said he would "take the same decision again."
Blair, whose popularity has plunged since his support for the US-led war on Iraq, conceded that his decision had hurt, angered and disappointed many of his supporters.
"Iraq has divided the international community. It has divided the party, the country, families, friends," he told delegates.
"I know many people profoundly believe the action we took
was wrong," Blair added.
Many Labour delegates resent Blair's decision to join the United States in invading Iraq, and the prime minister has been on the defensive because US-led forces have not found any of the weapons of mass destruction which were Blair's key argument for action.
"I ask just one thing: attack my decision but at least understand why I took it and why I would take the same decision again".
Former Foreign Secretary, Robin Cook, told a meeting on Monday that the war was disastrous.
Blair faces the most testing period of his six-year rule
"It is still possible to find colleagues who will defend the decision to invade Iraq," Cook said.
"It is very difficult to find anybody in the parliamentary party who doesn't recognise it has been a first-class political disaster for the party and the Labor government."
Blair faces the most testing period of his six-year rule. Doubts over his case for war have been fuelled by the failure to find any weapons of mass destruction in Iraq - Britain's main justification for military action.
A judicial inquiry into the suicide of weapons' expert, Dr David Kelly, who questioned parts of Britain's dossier on Iraqi weapons has further undermined the prime minister.
A poll released on Sunday found that 64% of Britons questioned last week said they did not trust Blair and 48% think he should resign.
The survey by ICM for the News of the World newspaper had a margin of error of three percentage points.