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The decision to go to war with Iraq was based on "flawed intelligence and assessments", the chairman of the Iraq Inquiry said.
"They were not challenged, and they should have been," said Sir John Chilcot.
The invasion of Iraq in 2003 has been described as the most politically divisive and internationally controversial war in British history.
Ahead of the release of the Chilcot Inquiry report, Sir John Chilcot told ITV News he hoped the report would mean nothing like the Iraq war ever happened again.
"The main expectation I have is that it will not be possible in future to engage in a military or indeed a diplomatic endeavor on such a scale and with such gravity without really careful challenge and analysis."
ITV News International Affairs Editor Rageh Omaar reports
Sending brave British troops into onto the battlefield in Iraq without proper equipment was "unacceptable", David Cameron said.
He added: "And whatever else we learn from this conflict, we must all pledge that this will never happen again."
Tony Blair has said that soldiers who died in the Iraq War did not die in vain, and that he disagrees that terrorism in the world today stems from the invasion.
The former prime minister added that today there is an elected government in Iraq, and that the world is a better place without Saddam Hussein.
Tony Blair should be brought to account over the Iraq War after the publication of the Chilcot report, the father of a soldier killed in the Iraq War has said.
Prominent anti-Iraq-War campaigner Reg Keys, whose 20-year-old son Thomas Keys, from Bala, in Gwynedd, died in Iraq, said lawyers will now be examining whether this was an illegal war.
He said he would like to see legal action against Mr Blair and other politicians who were involved in the decision to send troops to Iraq.
Mr Keys said the "ultimate goal" would be "to hold them to account in a court of law" but "publicly they've already been judged".
Jack Dromey, the MP for Birmingham Erdington has said the Iraq war has left a 'lasting bitter legacy.'
The Labour MP agreed with Sir John Chilcot's verdict, telling his followers on social media that profound lessons had to be learnt.
The decision to go to war in Iraq was the "hardest, most agonising decision" he ever made, said former prime minister Tony Blair, adding: "I accept full responsibility" for it.
Reg Keys, the father of Lance Corporal Thomas Keys, from Solihull has spoken to ITV News Central following the publication of the Iraq Report.
As well as other politicians at the time, who were involved in the decision to send our troops into the Middle East.
Latest ITV News reports
Thursday morning's newspapers were no less critical of Tony Blair than Sir John Chilcot's report into the Iraq war itself.
The former prime minister said he accepted full responsibility for the Iraq War failings - but insists he "did not mislead the country".