The findings of the 2.6 million-word Iraq Inquiry -- seven years in the making -- were released following a statement by probe chairman John Chilcot in London Wednesday.
The former civil servant said that Iraqi dictator Saddam Hussein posed "no imminent threat" when the U.S-led invasion was launched in March 2003, and that while military action against him "might have been necessary at some point," the "strategy of containment" could have continued for some time.
Chilcot said former British Prime Minister Tony Blair was warned of the risks of regional instability and the rise of terrorism before the invasion of Iraq, but pressed on regardless.
The UK failed to appreciate the complexity of governing Iraq, and did not devote enough forces to the task of securing the country in the wake of the invasion, he added.
"The people of Iraq have suffered greatly," he said.
Was it legal?
Blair's decision to invade Iraq was influenced by his interest in protecting the UK's relationship with the United States, the report said.
While the legal basis for the war was "far from satisfactory," the inquiry did not express a view on whether the invasion was legal, Chilcot said, arguing that that was a decision for another forum.
The inquiry was commissioned in June 2009 by Blair's successor, Gordon Brown, following pressure from the public and Parliament.
Charged with examining the build-up to the conflict, the war itself and its bloody aftermath -- over a period from 2001 to 2009 -- the inquiry was initially expected to take a year to complete.
Instead it has taken more than seven -- longer than the war itself -- with the final report running to 12 volumes.
Blair: World is safer because of invasion
In response to the scathing criticism, Blair said in a news conference Wednesday that he took full responsibility for the decision to invade Iraq, and acknowledged the failures and mistakes of the mission.
"I express more sorrow, regret and apology than you may ever know or believe," he said. "The intelligence assessments made at the time of going to war turned out to be wrong."
But while acknowledging "all the problems that came with that decision," Blair said, "what I cannot do and will not do is say I believe we took the wrong decision.
"I believe I made the right decision and that the world is better and safer as a result of it," he said.
In fact, Blair said, Iraq could have been in a worse predicament had Saddam remained in power, and turned the country into another Syria during the Arab Spring. Rather, Iraq had proved relatively stable during the Arab Spring, with ISIS only flourishing once ungovernable areas opened up in neighboring Syria, he said.
"The world was, and is in my judgment, a better place without Saddam Hussein," said Blair, insisting that the British service personnel who had lost their lives in the war had not died in vain.
He called on the world to mount a more coordinated response to confronting Islamist extremism, which he called "the scourge of our time."
Blair: No lies or deceit
Earlier in the day, he had noted in a statement that the report had found "no falsification or improper use of intelligence," "no deception of Cabinet," and "no secret commitment" between Blair and then-U.S. President George W. Bush to go to war.
He said that the Chilcot report "should lay to rest allegations of bad faith, lies or deceit. Whether people agree or disagree with my decision to take military action against Saddam Hussein; I took it in good faith and in what I believed to be the best interests of the country."
Alastair Campbell, Blair's influential communications director at the time, responded to the report with a bullish blog post
"Many mistakes yes, but no lies, no deceit, no secret deals, no 'sexing up.' And ultimately a matter of leadership and judgment," read the headline.
Calls for action against Blair
There have been increasing calls for Blair to face legal action over his role in taking Britain into the deeply unpopular war, the country's most controversial foreign policy decision of the modern era.
In a sign of the still-lingering anger over the invasion, anti-war protesters gathered outside the London office building where the report was released.
Lindsey German, a founding member of the Stop the War Coalition -- a group behind the mass demonstrations against the invasion in 2003 -- called the report a "damning indictment" and said that there "must be legal sanctions against Tony Blair and he should no longer be considered fit for any office."
Veterans' families: Loved ones died in vain
The families of some of the 179 British service personnel who lost their lives in the Iraq War said following the release of the report that they felt their loved ones had died for nothing.
"When I look at Iraq on our TV screens today, the 200-plus deaths that took place the other day, I can only conclude that unfortunately and sadly, my son died in vain," said Reg Keys, whose son died serving in Iraq, at a news conference.
Matthew Jury, a lawyer representing families who lost loved ones in the conflict, said his clients are "of course saddened that it appears to have been confirmed that their loved ones died unnecessarily and without just cause or purpose."
Sarah O'Connor, whose brother was killed in the war, said she was angry that "11 and a half years of healing" had been undone.
"I've gone back to that time when I learned that my brother had been killed." she said. "And there is one terrorist in this world that the world needs to be aware of, and it's Tony Blair -- the world's worst terrorist."
The families would be examining the report to see whether it forms the basis of any legal proceedings, they said.
Reaction from the Middle East
"People don't feel that the Iraq of today is much better than under Saddam Hussein," said CNN's Ben Wedeman, after asking people on the streets of Baghdad what they thought of Wednesday's report.
"Many feel that Tony Blair and George Bush should be put on trial for their botched (military) operation."
However, Wedeman added that some Kurds in the country were extremely supportive of the former British prime minister and U.S. president, and would even like to see the pair return to government.
"Opinion here is divided," said Wedeman. "Though you won't find too many kind words about Tony Blair."
Faisal Al Yafai, a reporter with Abu-Dhabi newspaper The National, said the real toll of the war could be seen in ordinary Iraqis.
"By failing to prepare, the U.S. and UK prepared to fail. And the consequences of that can be seen on the streets of Baghdad today," he said.
"When you look at those millions of words and hundreds of people interviewed, there was not one who talked about Iraq today -- and this is not a faraway war."
Russia: I told you so
Outgoing British Prime Minister David Cameron, who was in favor of the war at the time, said that despite the damning findings Britain would not retreat from taking military action in the future.
"It would be wrong to conclude that we shouldn't stand with our American allies when our common security interests are threatened," he said in Parliament.
He also praised the UK's intelligence services, and said while the country "cannot turn back the clock" on the Iraq War, it could learn lessons from it.
"Taking the country to war should always be a last resort, and should only be done if all credible alternatives have been exhausted," he said.
British Labour Party leader Jeremy Corbyn, who fervently opposed the war as one of Blair's backbenchers at the time, said the war was "an act of military aggression launched on a false pretext" and demonstrated the need for "stronger oversight of security and intelligence services."
Iraq's leaders have said they see the inquiry as an internal British matter, as they face more pressing security concerns at home in the wake of a deadly ISIS-claimed truck bombing that killed more than 250 people
in Baghdad last weekend.
Russia, which opposed the invasion of Iraq, responded through a tweet from its embassy in the UK.
"#Chilcot inquiry: No real WMD in Baghdad, unjust & highly dangerous war. The entire region on the receiving end," it wrote, attaching a graphic reading "Keep calm but I told you so."
Britain's Parliament approved the war -- ostensibly to remove Saddam Hussein and rid the country of weapons of mass destruction (WMD) -- shortly before the invasion, although United Nations approval was not gained and millions marched in the streets in protest.
Hussein was removed and later executed. But the WMD threat was found to have been overblown and the promise to turn a dictatorship into a democracy was never delivered on.
Instead, the country descended into years of vicious sectarian conflict, with large areas seized by the terror group ISIS.
More than 250,000 people have died violent deaths since the 2003 invasion, according to the Iraq Body Count project, while millions of Iraqis have been made homeless in the conflict with ISIS.