Former British prime minister Tony Blair took his country into a badly planned, woefully executed and legally questionable war in Iraq in 2003, according to the findings of a long-delayed inquiry published Wednesday.

The Chilcot report said Britain joined the US-led invasion before all other options had been exhausted and on the basis of false intelligence that Iraqi leader Saddam Hussein had weapons of mass destruction.

Blair faced particular criticism for pledging to support US president George W. Bush the year before the invasion, writing: "I will be with you, whatever".

Blair failed to ensure "there was a flexible, realistic and fully resourced plan", said the report, which found preparations for occupation after the initial invasion were "woefully inadequate".

More than 150,000 Iraqis had died by the time most British troops withdrew in 2009, while 179 British soldiers also lost their lives.

The country remains plagued by sectarian violence, as shown notably by Sunday's Baghdad suicide bombing claimed by Islamic State which killed at least 250 people.

In a statement, Blair said he had acted "in good faith and in what I believed to be the best interests of the country".

He said he took "full responsiblity for any mistakes, without exception or excuse", but added: "I believe that it was better to remove Saddam Hussein."

The inquiry drew a different conclusion.

It found that "military action in Iraq might have been necessary at some point. But in March 2003 there was no imminent threat from Saddam Hussein."

Britain's scarring experience in Iraq has made it deeply wary of committing ground troops to international military interventions in countries like Syria and Libya.

Unveiling the 2.6 million-word report, which took seven years to complete, inquiry chairman John Chilcot said it was "an account of an intervention which went badly wrong, with consequences to this day".

More than 100 anti-war protesters gathered outside the conference centre where the report was published, with many shouting: "Blair lied, thousands died" and "war criminal Tony Blair".

Relatives of some of the soldiers killed said the report could form the basis of legal action against Blair and other officials.

"The inquiry has confirmed all our fears that these young men and women were deployed on the back of a falsehood," said Roger Bacon, whose son Matthew, 34, died in 2005.

"We reserve our right to call specific parties to answer for their actions in the courts, if such process is found to be viable."

Rose Gentle, who lost her 19-year-old son Gordon, said the findings were "gut-wrenching" -- and dismissed Blair's response.

"I hold him responsible for the murder of my son," she told reporters.

- Peaceful options not exhausted -

Although the legality of the invasion was not in his remit, retired civil servant Chilcot said the process of deciding the legal basis for war was "far from satisfactory".

"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," he said.

The report laid the blame for mistaken fears of Saddam's WMD firmly on the intelligence community, but said the government had "overstated the firmness of the evidence" about Iraq's capabilities and intentions.

It confirmed long-held suspicions that Blair put Britain on a path to war as early as July 2002.

Blair was also criticised for failing to challenge Bush on the lack of planning for the aftermath of the invasion.

It dismissed Blair's assertion that it was not possible to predict the strength of local opposition, the rise of Al-Qaeda and the involvement of Iran, which all fuelled the violence, saying these were "explicitly identified before the invasion".

- 'Lessons learned' -

The inquiry was called under pressure from relatives, concerned about the justification for the war as well as poor military equipment, over which the Ministry of Defence was strongly criticised in the Chilcot report.

The families are not the only ones considering legal action against Blair -- a cross-party group of MPs is also looking into the possibility, including of taking a case to the International Criminal Court.

The war, which at one point saw 46,000 British troops deployed, mostly in southern Iraq around the strategic oil hub of Basra, still looms large over British politics.

In a statement to the House of Commons, Prime Minister David Cameron said that all the MPs who voted for the war must "take our fair share of the responsibility.

"We cannot turn the clock back but can ensure that lessons are learned and acted on," he said.

Cameron said new procedures to ensure "proper separation" between intelligence and the process for assessing has already been put in place.

"Taking the country to war should always be a last resort," he said, adding however that "we should not conclude that intervention is always wrong".

Opposition leader Jeremy Corbyn, the current head of Blair's Labour party, said the war was "an act of military aggression launched on a false pretext" that "fuelled and spread terrorism across the region".