Tony Blair overstated the threat posed by Saddam Hussein, sent ill-prepared troops into battle and had "wholly inadequate" plans for the aftermath, the UK's Iraq War inquiry has said.
Chairman Sir John Chilcot said the 2003 invasion was not the "last resort" action presented to MPs and the public.
There was no "imminent threat" from Saddam - and the intelligence case was "not justified", he said.
Mr Blair apologised for any mistakes made but not the decision to go to war.
The report, which has taken seven years, is on the Iraq Inquiry website.
Prime Minister David Cameron, who voted for war in 2003, told MPs it was important to "really learn the lessons for the future" and to improve the workings of government and how it treats legal advice.
And he added: "Sending our brave troops on to the battlefield without the right equipment was unacceptable and, whatever else we learn from this conflict, we must all pledge this will never happen again."
Labour leader Jeremy Corbyn - who voted against military action - said the report proved the Iraq War had been an "act of military aggression launched on a false pretext", something he said which has "long been regarded as illegal by the overwhelming weight of international opinion".
After meeting relatives of British service people killed in Iraq, Mr Corbyn said: "I now apologise sincerely on behalf of my party for the disastrous decision to go to war."
He urged the UK to back moves to give the International Criminal Court "the power to prosecute those responsible for the crime of military aggression".
A spokesman for some of the families of the 179 British service personnel and civilians killed in Iraq between 2003 and 2009 said their loved ones had died "unnecessarily and without just cause and purpose".
He said all options were being considered, including asking those responsible for the failures identified in the report to "answer for their actions in the courts if such process is found to be viable".
In a statement to the media, his voice at times cracking with emotion, the former Labour prime minister said the decision to commit troops was the "most agonising and momentous" decision in his decade as prime minister, adding that he would "carry it with me for the rest of my days".
"I feel deeply and sincerely in a way that no words can properly convey the grief and sorrow of those who lost ones they loved in Iraq - whether our armed forces, the armed forces of other nations or Iraqis.
"The intelligence assessments made at the time of going to war turned out to be wrong, the aftermath turned out to be more hostile, protracted and bloody than ever we imagined.... and a nation whose people we wanted to set free from the evil of Saddam became instead victims of sectarian terrorism.
"For all of this, I express more sorrow, regret and apology than you may ever know or can believe."
But he was defiant on the central decision to go to war, saying "there were no lies, Parliament and Cabinet were not misled, there was no secret commitment to war, intelligence was not falsified and the decision was made in good faith".
It's been a long wait.
It may prove to have been a worthwhile wait for the people who have always opposed the Iraq War.
Remember, one million individuals took to the streets in 2003 in opposition to the march to war.
They will seize on this Inquiry's judgement that Saddam Hussein didn't pose an immediate threat and military action at that time was not a last resort.
Those seeking action against Tony Blair are likely to be disappointed - but probably not that surprised - that a panel which didn't include any lawyers, hasn't expressed a view on whether military action was legal.
Sir John Chilcot's public remarks were peppered with the word "failure".
But he was careful not to apportion blame.
Others will now do that on the evidence his report has placed in the public domain.
The political space will be filled with claims and counter claims about a war in Iraq where - as Sir John Chilcot put it - its people have suffered greatly.
In a nearly two hour news conference he said he would never agree that those who died or were injured in Iraq "made their sacrifice in vain" as they had played their part in "the defining global security struggle of the 21st century against the terrorism and violence which the world over destroys lives, divides communities".
Quizzed about what he was apologising for, he said: "There is no inconsistency in expressing my sorrow for those that have lost their lives - my regret and my apology for the mistakes - but still saying I believe the decision was right. There is no inconsistency in that."
He said the US would have launched an invasion "either with or us or without us", adding: "I had to decide. I thought of Saddam and his record, the character of his regime. I thought of our alliance with America and its importance to us in the post 9/11 world and I weighed it carefully with the heaviest of hearts."
Mr Blair, who was PM from 1997 to 2007, conceded that intelligence on Iraq's weapons had "turned out to be wrong" and the invasion had destabilised Iraq but said he still believed the country was "better off" without Saddam, comparing it with the situation in Syria where the decision had been taken not to intervene.
He also said he should have "disclosed" the attorney general's legal advice to the Cabinet on the eve of war - but he defended his close relationship with President Bush, saying: "we are better to be strongly onside with the US", arguing that it was "better for our own security".
George Bush comments
George W Bush's communications director, Freddy Ford, told BBC News: "President Bush is hosting wounded warriors at his ranch today and has not had the chance to read the Chilcot report.
"Despite the intelligence failures and other mistakes he has acknowledged previously, President Bush continues to believe the whole world is better off without Saddam Hussein in power.
"He is deeply grateful for the service and sacrifice of American and coalition forces in the war on terror. And there was no stronger ally than the United Kingdom under the leadership of Prime Minister Tony Blair.
"President Bush believes we must now find the unity and resolve to stay on the offensive and defeat radical extremism wherever it exists."
Sir John, the ex-civil servant who chaired the inquiry, describes the Iraq War as an intervention that went "badly wrong" with consequences still being felt to this day - and he set out lessons to be learned for future conflicts.
His report, which is 2.6 million words, does not make a judgement on whether Mr Blair or his ministers were in breach of international law.
In his statement, Sir John said military action against Saddam Hussein might have been necessary "at some point" but that when Britain joined the US-led invasion in March 2003, the Iraqi dictator posed "no imminent threat", the existing strategy of containment could be continued and the majority of UN Security Council members supported continuing UN inspections and monitoring".
He added: "The judgements about the severity of the threat posed by Iraq's weapons of a mass destruction - WMD - were presented with a certainty that was not justified. Despite explicit warnings, the consequences of the invasion were underestimated."
Previously classified documents, including 31 personal memos from Tony Blair to then US president George W Bush, have been published alongside the Chilcot Report.
They show that momentum in Washington and London towards taking action against Saddam Hussein quickly began to build in the wake of the 9/11 attacks in 2001 in the US, which killed nearly 3,000 people.
On the day after the attack on New York's Twin Towers, Mr Blair sent a note to President Bush offering his support to bring to justice the hijackers and looked ahead to the "next stage after this evil".
Mr Blair said some would "baulk" at the measures necessary to control "biological, chemical and other weapons of mass destruction", but added: "We are better to act now and explain and justify our actions than let the day be put off until some further, perhaps even worse, catastrophe occurs."
The memos reveal that Mr Blair and Mr Bush were openly discussing toppling Saddam Hussein as early as December 2001, when the UK and US had just launched military action in Afghanistan.
"How we finish in Afghanistan is important to phase 2. If we leave it a better country, having supplied humanitarian aid and having given new hope to the people, we will not just have won militarily but morally; and the coalition will back us to do more elsewhere," says Mr Blair in the memo.
"We shall give regime change a good name which will help in our arguments over Iraq."
In another memo, from July 2002 - nearly a year before the invasion of Iraq - Mr Blair assured President Bush that the UK would be with him "whatever," but adds that if Mr Bush wanted a wider military coalition he would have to get UN backing, make progress on Middle East peace and engineer a "shift" in public opinion in the US, UK and the Arab World.
The note, marked "personal," was shared with then Foreign Secretary Jack Straw, but not then defence Secretary Geoff Hoon - a decision criticised by Sir John, who is scathing about the way the collective Cabinet discussion was bypassed by the Blair government.