The 2003 Iraq War was launched on the back of flawed intelligence, the official seven-year-long probe into Britain's invasion has found.
Claims about the threat posed by weapons of mass destruction (WMD) were also presented to MPs with a “certainty that was not justified”, the inquiry's chair Sir John Chilcot said.
Tony Blair also "overestimated his ability to influence US decisions" on Iraq, while his government's planning for the immediate aftermath of the War was “wholly inadequate”, he said.
Sir John also hit out at the far "from satisfactory” process that led ministers to conclude there was a legal basis for the war.
Read more: Chilcot report: Tony Blair rushed ill-prepared troops to war with no imminent threat
The former Whitehall mandarin said from the outset that his inquiry would make no judgement on whether or not the invasion decision was legal in terms of international law.
Instead he said his investigation would provide a "full and insightful" account of the decision-making.
Mr Blair has been accused of pledging to join the war during a meeting with then US President George Bush in Texas in 2002.
Questions marks have also hung over the eleventh hour decision by then Attorney General, Lord Goldsmith, to change his mind and advise the government that action would be legal.
The 12-volume Iraq Inquiry report looked at UK policy from 2001 to 2009.
Sir John presented his findings in front of relatives of some of the 179 British service personnel who lost their lives.
He also criticised equipment shortfalls and said that the Ministry of Defence had been slow to react to deadly improvised explosive devices (IEDs).
Sir John said: “We have concluded that the UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted.
“Military action at that time was not a last resort.”
In March 2003 there was no imminent threat from Saddam Hussein and most countries on the UN Security Council supported continuing inspections.
Sir John's report also accused the UK of "undermining" the security council’s authority in the drive for war.
In September 2002, Mr Blair had told MPs in the Commons about Iraq’s capabilities.
Sir John said Mr Blair's comments were presented “with a certainty that was not justified”.
The then prime minister's advisers should have made clear to him that intelligence had not established ‘beyond doubt’ if Iraq continued to produce chemical and biological weapons or if it was still trying to develop nuclear weapons.
Sir John also hit out saying that "hindsight" was not required to foresee a number of the problems that would afflict Iraq in the aftermath of the invasion.
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