Tony Blair convinced himself with unjustified certainty that Iraq’s President, Saddam Hussein, had weapons of mass destruction, when intelligence reports had not established "beyond doubt" that they existed, the long awaited Chilcot report has damningly concluded.

The Prime Minister was so convinced that of the presence of the non-existent WMDs that he sent British troops into Iraq when diplomacy might still have resolved the crisis. But the secret intelligence reports he had been shown "did not justify" his certainty, Sir John Chilcot concluded.

Sir John Chilcot's damning report into the Iraq War also revealed that Blair and US President George W. Bush were made fully aware that Iraq could descend into sectarian chaos after the invasion – directly contrary to what Mr Blair told the inquiry.

The issue of whether the then Prime Minister lied to Parliament to justify the UK’s involvement in the Iraq war has been a source of damaging controversy for more than 13 years.

Sir John Chilcot did not use the word “lie” – in fact his report specified that it “is not questioning” Mr Blair fixed belief - but his damning conclusion is that the former Prime Minister deliberately blurred the distinction between what he believed and what he actually knew.

However, Mr Blair claimed that the 12 volume report proved that, at worst, he had made an honest mistake. He said, in a statement: “The 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.”

Sir John also said the risks of internal strife, regional instability and the burgeoning of al-Qaeda in Iraq "were each explicitly identified", yet planning and preparations for Iraq after Saddam were "wholly inadequate".

And he criticised intelligence chiefs for allowing the Prime Minister to get away with misrepresenting what they had told him when he presented his now notorious dossier to the House of Commons in September 2002.

The dossier itself, which accurately reflected what the intelligence services knew, came with a foreword signed by Tony Blair, which claimed that it established “beyond doubt” that Iraq held WMDs. This was a “deliberate selection of a formulation which grounded the statement in what Mr Blair believed, rather than in the judgements which the JIC had actually reached,” Sir John’s report concluded.

The key players in the Iraq War Show all 11 1 /11 The key players in the Iraq War The key players in the Iraq War Jack Straw Jack Straw was the UK foreign secretary at the time of the Iraq invasion, and fully endorsed the decision Getty The key players in the Iraq War Geoff Hoon Geoff Hoon was Tony Blair’s defence secretary from October 1999 to May 2005 Getty Images The key players in the Iraq War Alastair Campbell Alastair Campbell was involved in the drafting of two Downing Street dossiers on the war, in September 2002 and in February 2003 Getty The key players in the Iraq War John Scarlett John Scarlett was chairman of the Joint Intelligence Committee at the time of the 2003 invasion Getty Images The key players in the Iraq War Peter Goldsmith Peter Goldsmith was Mr Blair’s attorney general from 2001 to 2007 AFP/Getty Images The key players in the Iraq War Condoleezza Rice Condoleezza Rice was named as National Security Advisor to George W Bush in 2000, becoming the first woman to occupy the post, and argued publicly in favour of the 2003 invasion Getty Images The key players in the Iraq War Colin Powell Colin Powell was Secretary of State from 2001 to 2005 Getty Images for TIME The key players in the Iraq War Tommy Franks Tommy Franks was the leading US general at the start of the Iraq war The key players in the Iraq War Dick Cheney Dick Cheney was George W Bush’s vice president from 2001 to 2009 Getty Images The key players in the Iraq War Paul Bremer Paul Bremer ran Iraq for 14 months after the invasion, appointed Bush’s Presidential Envoy in charge of the occupying forces Getty Images The key players in the Iraq War Hans Blix Hans Blix was the UN weapons inspector tasked with monitoring Iraq from 2002 to 2003 Getty Images

“The judgements about Iraq’s capabilities in that statement, and in the dossier published the same day, were presented with a certainty that was not justified,” he said.

“The Joint Intelligence Committee should have made clear to Mr Blair that the assessed intelligence had not established ‘beyond doubt’ either that Iraq had continued to produce chemical and biological weapons or that efforts to develop nuclear weapons continued.”

But he acquitted Tony Blair’s director of communications, Alastair Campbell, of the charge that he “sexed up” the September dossier to distort the intelligence – an allegation aired on the Today programme in May 2003 by the journalist Andrew Gilligan, who claimed to have heard from the weapons inspector Dr David Kelly. “There is no evidence that intelligence was improperly included in the dossier or that No 10 improperly influenced the text,” the report concluded.

Sir John’s report damningly added that as the prospect loomed that the US was going to invade Iraq whatever the British decided, the intelligence chiefs gave “no consideration” to the possibility that Saddam Hussein was – for once –telling the truth when he said that his regime had destroyed all the chemical weapons it possessed and used in the 1980s.

The Iraq government announced in November 2002 - four months before the invasion - that it no longer had any weapons of mass destruction, but Mr Blair refused to believe that. Speaking on the telephone to President George W Bush the following month he said that the Iraqi declaration was “patently false” and that he was “cautiously optimistic” that weapons inspector would be able to prove that the Iraqis were lying.

Kim Sengupta reflects on his time in Iraq ahead of the Chilcot report

On 18 March, 2003, Tony Blair persuaded the House of Commons to give the go ahead for military action, but “at the time of the parliamentary vote, diplomatic options had not been exhausted,” Sir John concluded.

“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."

From September 2002, six months before the invasion, Foreign Office and intelligence reports indicated the war would create an "easier environment for terrorists" and the destabilisation of the country.

An FCO paper on Islamism in Iraq, shared with the Americans in December 2002, even foreshadowed the rise of extremist groups like Isis which went on to exploit the chaos of post-war Iraq.

It warned that it was likely groups would be looking for “identities and ideologies on which to base movements” and anticipated that a number of emergent extremist groups would use violence to pursue political ends.

Isis, which 11 years after the invasion declared a caliphate in Iraq, remains in control of vast swathes of the country, including its second city Mosul. The group claimed responsibility for Sunday’s bombing in Baghdad, the death toll of which has now risen to 250 – the worst such attack since the invasion in 2003.

“Mr Blair told the inquiry that the difficulties encountered in Iraq after the invasion could not have been known in advance,” Sir John said, as he released the report in London on Wednesday morning.

“We do not agree that hindsight is required. The risks of internal strife in Iraq, active Iranian pursuit of its interests, regional instability, and al Qaeda activity in Iraq, were explicitly identified before the invasion.”