Former prime minister Tony Blair said he would take the same decision again to join the US-led 2003 invasion of Iraq, after he and other officials were accused of misleading the British public and parliament over the intelligence used to justify the war.

Blair was reacting to an inquiry report that said Britain had decided to invade Iraq before exhausting peaceful options, used intelligence presented with a "certainty that was not justified," and undermined the authority of the United Nations.

"We have concluded that the UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted," said John Chilcot, lead author of the 2.6-million-word report, which presented the results of an inquiry that took seven years to complete.

"Military action at that time was not a last resort," Chilcot said.

"The judgements about the severity of the threat posed by Iraq's weapons of mass destruction - WMD - were presented with a certainty that was not justified."

Blair admitted "mistakes in planning and process" in Britain's role in the invasion, but insisted: "I would take the same decision [again]."

He decided to invade Iraq "because I believed it was the right thing to do, based on the [available] information."

Blair's cabinet agreed to invade Iraq if Saddam Hussein did not accept a final US ultimatum to leave within 48 hours, in March 2003. Parliament backed the decision the following day.

"The decision was, however, shaped by key choices made by Mr Blair's government over the previous 18 months," Chilcot said.

In a July 2002 note on Iraq, Blair told US President George W Bush: "I will be with you, whatever."

On Wednesday, Blair said the note to Bush was designed to "make it clear I was going to be with the Americans" but was not an open agreement for Washington to go ahead with the invasion, as some critics have claimed.

By December 2002, Bush had decided that UN weapons inspections "would not achieve the desired result" and the United States would take military action in early 2003, the Chilcot report said.

Blair accepted a US timetable for military action by mid-March 2003, and Bush agreed to help him by seeking a further UN Security Council resolution on Iraq, the report said.

But it became clear it would not be possible to persuade a majority of the Security Council to support a second resolution before the US took military action.

"In the absence of a majority in support of military action, we consider that the UK was, in fact, undermining the Security Council's authority," Chilcot said.

Blair rejected that conclusion. "The US was going to war, with us or without us," he told reporters during two-hour defence of his conduct.

"The reality is that we - Britain - had continuously tried to act with the authority of the UN. I made the decision in good faith on the information I had at the time."

Blair insisted that the world was "a better place" after the fall of Saddam Hussein's regime. "What I cannot do, and will not do, is say we took the wrong decision," he said.

The White House said it had not yet fully evaluated the entire report, but spokesman Josh Earnest noted the close cooperation between the US and British governments had continued across administrations.

"The ability of the leaders of our countries to work together to focus on our common interests and to pursue them jointly has made our countries more prosperous and more safe," he said, noting that President Barack Obama had been opposed to the Iraq invasion and had had to deal with the consequences on taking office in 2009.

Blair said the invasion was the "hardest, most momentous and agonizing" decision of his time as prime minister.

He admitted that the analysis of intelligence on Saddam Hussein's development of weapons of mass destruction "turned out to be wrong," and said the aftermath of the invasion was "more bloody than we ever imagined."

Chilcot said the invasion "failed to achieve its stated objectives," but Blair rejected claims it is "the cause of the terrorism we see today whether in the Middle East or elsewhere in the world."

Scores of anti-war protesters gathered in central London as the report was released, some calling for Blair to be prosecuted for war crimes and many carrying placards bearing the word "Bliar."

Blair insisted there were "no lies, no deceit" in his conduct.

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