Tony Blair says he "will take full responsibility for any mistakes without exception or excuse"
John Chilcot, the head of the Iraq Inquiry, says British military action in Iraq was not a last resort.
The long-awaited report of the inquiry has been published this morning. The full document is available here.
The inquiry has not expressed a view on whether or not the war was legal - claiming that can only be decided by the courts.
The chair described the choice to invade Iraq as "a decision of the utmost gravity".
He says the decision to invade was made "before the peaceful option for disarmament had been exhausted".
The severity of the threat posed by Iraq's weapons of mass destruction "were presented with a certainty that was not justified" by Tony Blair's government, Mr Chilcot added.
Mr Blair says he will make a full statement on the findings later today.
"I will take full responsibility for any mistakes without exception or excuse," the former UK Prime Minister said.
"I will at the same time say why, nonetheless, I believe that it was better to remove Saddam Hussein and why I do not believe this is the cause of the terrorism we see today whether in the Middle East or elsewhere in the world," he added.
The Iraq Inquiry found the consequences of the 2003 invasion were underestimated "despite explicit warnings".
The circumstances in which it was decided there was a legal basis for UK military action in Iraq were "far from satisfactory", the inquiry found. The invasion was based on "flawed intelligence" which was not challenged, but "should have been".
There was "little time" to properly prepare three military brigades for deployment, and the risks were neither "properly identified nor fully exposed" to ministers. The response to the threat of improvised explosive devices (IEDs) was also slow, he said.
Mr Blair was warned that military action in Iraq would increase the threat of al Qaeda to the UK.
The planning and preparation for the period after Saddam Hussein's fall was "wholly inadequate", Mr Chilcot observed.
He said that the UK military option in Iraq ended a very far way from success. It was an "intervention that went badly wrong, with consequences felt to this day."
However, he also stated: "Service personnel, civilians who deployed to Iraq and Iraqis who worked for the UK, shows great courage in the face of considerable risks. They deserve our gratitude and respect".
Announcing two days of House of Commons debates on the report next week, David Cameron said: "The most important thing to do is to clearly learn the lessons for the future, and the lessons he lays out quite clearly.
"If we are to take difficult decisions to intervene in other countries, then proper planning is vital."
Labour leader Jeremy Corbyn said the report showed the war was "an act of military aggression launched on a false pretext" and said the conflict has long been regarded as illegal.
He added that the "flimsy and conflicting" evidence on WMDs had in part led to a "fundamental breakdown in trust in politics".
He did not mention Tony Blair by name, but has a speech scheduled later today when he may do so.
Alan Simpson, Former British Labour MP, told Newstalk Lunchtime that the UK parliament now has two options.
"We now have families in the UK whose sons, daughters, husbands, brothers died on the basis of a lie," he argued.
"[Parliament] could revoke the ennoblement of Blair and any other ministers who were really responsible for this. Second, they could refer this to the International Criminal Court, because in almost any other country that is what you'd expect to be done," he added.
Protests were held in central London as Mr Chilcot unveiled his report.
Ex-soldiers who served in Iraq joined leaders of the Stop the War Coalition, CND and other groups to demand "truth and justice".
Lindsey German of the Stop the War Coalition said: "Chilcot should not be the end of the matter - it must be the beginning of legal and political action against Blair."
The long-awaited report is being published seven years after the Iraq Inquiry began and 13 years after British troops crossed into Iraq.
The relatives of the 179 men and women who died in Iraq got advance sight of the report, which runs to 2.6 million words.
The report has made public never-seen-before correspondence between the UK and US leaders in the run-up to the war.