A 12-volume report into the invasion of Iraq has issued a damning indictment of the decision to go to war, saying it was based on false information and laid the groundwork for much of the turmoil that plagues the region today.

Britain's Iraq Inquiry spent seven years examining whether it was necessary to invade Iraq in 2003 to remove Saddam Hussein and whether Britain and its allies should have been better prepared for the aftermath.

The report, released Wednesday, concluded there were other options to war and that the legal basis for invading was "far from satisfactory." It also said the planning for a postwar Iraq was woefully inadequate and that the resulting instability resulted in the death of at least 150,000 Iraqis "and probably many more."

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"The evidence is there for all to see. It is an account of an intervention which went badly wrong, with consequences to this day," said inquiry chairman Sir John Chilcot. "Military action at that time was not a last resort."

The report has reopened a long-simmering debate about Britain's role in Iraq, the conduct of former prime minister Tony Blair and the country's standing in the world. And it comes amid rising violence in Iraq, Syria and elsewhere in the region that many say can be traced back to the 2003 invasion and the failure of the U.S. and Britain to rebuild Iraq and support the country's fledgling administration.

Mr. Blair spent two hours on Wednesday defending his actions. He said he regretted the lost lives and acknowledged mistakes were made, but insisted he made the right call to invade.

"Please stop saying I was lying or I had some dishonest or underhand motive," Mr. Blair said. "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."

He added that he had no regrets about backing up then U.S. president George W. Bush, saying Britain has to work in coalitions and the U.S. is the country's most important ally.

"The inquiry claims that military action was not a last resort, but it also says that it might have been necessary later. With respect, I didn't have the option of that delay," Mr. Blair said. "I took this decision because I believed it was the right thing to do based upon the information I had and the threats I perceived."

Family members of British soldiers who died in Iraq heaped scorn on Mr. Blair and threatened to sue.

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"There's one terrorist in this world, that the world needs to be aware of, and his name is Tony Blair," said Sarah O'Connor, whose brother, Sergeant Bob O'Connor, died in 2005 when the helicopter he was in was shot down.

"My son died in vain," said Roger Bacon, who lost his son, Major Matthew Bacon, during the war.

Rose Gentle, whose son, Gordon, died in a roadside bomb blast in 2004, said she wanted to look Mr. Blair in the eye and ask him: "Why did you kill my son?"

The inquiry's conclusions will deepen the political divisions in Britain, particularly in the Labour Party where leader Jeremy Corbyn has been a fierce critic of Mr. Blair, a Labour prime minister for 10 years, and is battling a revolt by so-called Blairites within the party.

The report examined the period immediately after the Sept. 11, 2001, terrorist attacks in the U.S. up to 2009 when British troops left Iraq.

Documents released by the inquiry, including dozens of personal notes between Mr. Blair and Mr. Bush, reveal that from the start Mr. Blair was keen to stand "shoulder to shoulder" with the U.S.

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"I will be with you, whatever," Mr. Blair said in a note to Mr. Bush in July, 2002. In several notes Mr. Blair expressed concern about the aftermath of the war and at one point he raised the possibility of installing another dictator until order could be restored.

The two men developed an extremely close working relationship, with Mr. Blair frequently sending the president detailed reports and injecting his views on major issues of the day. Mr. Bush, the report said, "encouraged that dialogue and listened to Mr. Blair's opinions."

Mr. Blair "had a habit of writing notes, both internally and to President [Bill] Clinton and President Bush on all sorts of subjects," Jonathan Powell, Mr. Blair's chief of staff, told the inquiry.

Britain was eager to back the U.S. immediately after the 9/11 attacks in part because it feared that vital areas of co-operation would be damaged unless it gave the Americans full support, the report found. The British government also thought that it could best influence U.S. policy by committing full support and seeking to persuade policy-makers from the inside.

"The issue of influencing the U.S., both at the strategic and at the operational level, was a constant preoccupation at all levels of the U.K. government," the report said.

However, the report concluded that while British arguments at times made a difference in shaping U.S. policy "the relationship between the two is unequal." And it noted that countries like Germany and France opposed invading Iraq and that "does not appear to have had a lasting impact on the relationships of those countries with the U.S., despite the bitterness at the time."

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The report also goes into detail about the drafting of a dossier Mr. Blair presented to Parliament in September, 2002, which claimed the "assessed intelligence" had "established beyond doubt" that Saddam Hussein had "continued to produce chemical and biological weapons, that he continues in his efforts to develop nuclear weapons, and that he had been able to extend the range of his ballistic missile program."

The dossier had been based on information from the Joint Intelligence Committee (JIC) which the inquiry said had not come to those conclusions at all. The JIC had found that Iraq had produced chemical and biological agents, and that the country had the means to deliver chemical weapons but "it did not say that Iraq had continued to produce weapons." It also made clear that as long as sanctions remained effective "Iraq could not produce a nuclear weapon." However the report said the JIC did not press its case with Mr. Blair, allowing him to come to his own conclusions.

The report said Mr. Blair had also been warned by his officials that military action would increase the threat from al-Qaeda and that an invasion of Iraq might lead to Iraq's weapons and capabilities being transferred into the hands of terrorists.

"It is now clear that policy on Iraq was made on the basis of flawed intelligence and assessments. They were not challenged, and they should have been," Sir John said.

The widespread perception that the dossier overstated the evidence in order to influence public opinion "has produced a damaging legacy, including undermining the trust and confidence in government statements, particularly those which rely on intelligence which cannot be independently verified," the report concluded.

The report said the British military was ill-prepared for the 2003 invasion, lacked proper equipment and there was scant planning for postwar Iraq.

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"The government's preparations failed to take account of the magnitude of the task of stabilizing, administering and reconstructing Iraq, and of the responsibilities which were likely to fall to the U.K.," the report said.