Britain joined the 2003 US-led invasion of Iraq before exhausting peaceful options, used intelligence presented with a "certainty that was not justified" and undermined the authority of the United Nations, an inquiry concluded on Wednesday.
Former prime minister Tony Blair and other British officials have come under fire for allegedly misleading the public and parliament over the intelligence behind the decision to take part in the Iraq invasion.
"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, the 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'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."
By December 2002, Bush had decided that UN weapons inspections "would not achieve the desired result" and the US would take military action in early 2003.
Blair accepted a US timetable for military action by mid-March 2003, the report said.
"To help Mr Blair, President Bush agreed to seek a further UN resolution - the 'second' resolution - determining that Iraq had failed to take its final opportunity to comply with its obligations," Chilcot said.
But it became clear it would not be possible to persuade a majority to support a second UN Security Council resolution before the US took military action.
Blair and his foreign secretary, Jack Straw, blamed France for a supposed impasse at the UN and claimed that Britain was acting on behalf of the international community to uphold the Security Council's authority.
"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.
Responding to the report, Blair insisted his decision to join the invasion was taken "in good faith and in what I believed to be the best interests of the country."
He conceded there were "serious criticisms and they require serious answers," promising to "respond in detail" later Wednesday.
"I will take full responsibility for any mistakes without exception or excuse," Blair said.
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."