Tony Blair has come under fire after claiming the violent insurgency in Iraq is not the result of the 2003 invasion to topple Saddam Hussein.
In a renewed call for military action, the former prime minister said the crisis unfolding in the country was instead the "predictable" result of the West's failure to intervene in Syria.
But his comments have met with scathing criticism from figures across the political spectrum, including his former deputy prime minister Lord Prescott and ex-international development secretary Clare Short, who resigned from her post over Iraq.
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In an eight-page essay on his website, Blair - now a Middle East peace envoy - rejected as "bizarre" arguments that Iraq would be more stable and peaceful today if the US-backed war, which claimed the lives of 179 UK personnel, had not happened.
He added Iraq was "in mortal danger", pinning the blame on the sectarianism of the Nouri al-Maliki government and the spread of Syria's brutal three-year civil war.
"The choices are all pretty ugly, it is true. But for three years we have watched Syria descend into the abyss and as it is going down, it is slowly but surely wrapping its cords around us, pulling us down with it," he wrote.
He made clear it did not mean another invasion, insisting there were a whole myriad of responses between troops on the ground and doing nothing at all.
"I understand all the reasons following Afghanistan and Iraq why public opinion was so hostile to involvement," Blair continued.
"Action in Syria did not and need not be as in those military engagements. But every time we put off action, the action we will be forced to take will be ultimately greater."
Speaking on the Andrew Marr Show, Blair reiterated his regret over the loss of life during the conflict, but insisted removing Saddam had been the right thing to do.
His intervention in the debate comes amid international moves to tackle the bloody insurgency by Islamist extremists and deal with its humanitarian consequences.
Thousands have fled the sweeping advance of fighters from the al-Qaida-inspired Islamic State of Iraq and the Levant (Isis), who have taken control of large areas including second city Mosul.
Isis, which captured two major cities last week, has posted graphic photos that appear to show its fighters massacring dozens of captured Iraqi soldiers, but government forces are reportedly beginning to win territory back.
US President Barack Obama is considering a range of military options - said to include air strikes - and on Saturday the US sent an aircraft carrier to the Gulf.
Iran - a key ally of al-Maliki's Shia-dominated administration in Baghdad - has also indicated that it is ready to provide assistance and Britain is to give £3 million of aid to Iraq.
Reacting to the essay, Ms Short said Blair had been "absolutely, consistently wrong, wrong, wrong" on the issue, and branded him a "complete American neocon".
Speaking on Murnaghan, she criticised the "deceit" over the decision to invade, and added: "More bombing will not solve it, it will just exacerbate it."
Lord Prescott said on the same programme that further intervention risked a return to the Crusades, adding: "I don't agree with Tony as I didn't then.
"He says we do it (intervene) where there isn't open economy or open society. What he means is western democracy."
Ukip leader Nigel Farage dismissed Blair as an "embarrassment" who should hold his tongue - and added he is opposed to Western military intervention in Iraq or Syria.
"The lesson is not, as Blair implies, that the West should intervene in Syria, let alone once more in Iraq. The lesson is that the West should declare an end to the era of military intervention abroad," he said.
Scotland's First Minister Alex Salmond added his voice to the chorus accusing Blair of "breathtaking amnesia" over his reasons for invading Iraq.
He said: "Tony Blair has now claimed that the invasion of Iraq was about whether or not Saddam Hussein remained in power. Eleven years ago he said it was about weapons of mass destruction.
"No reinterpretation of history will absolve the former prime minister of a direct line of responsibility for this sequence of disasters."
Similarly Sir Christopher Meyer, who was Britain's ambassador to the US from 1997 to 2003, said Blair was wrong and that the handling of the campaign against Saddam was "perhaps the most significant reason" for the sectarian violence now ripping through Iraq.
"We are reaping what we sowed in 2003. This is not hindsight. We knew in the run-up to war that the overthrow of Saddam Hussein would seriously destabilise Iraq after 24 years of his iron rule," he wrote in the Mail on Sunday. "For all his evil, he kept a lid on sectarian violence."
Former Labour foreign office minister Lord Malloch-Brown said Blair should "stay quiet", adding: "You know, Tony Blair is half right...Iraq, like Syria, would probably have been a problem even without an intervention.
"But one wishes someone would tell him to just stay quiet during moments like this, because it does drive a great surge of people in the other direction."
Former Liberal Democrat leader Lord Ashdown told Murnaghan he too was "firmly interventionist", but said: "I don't believe it is right in these circumstances in the way that Tony suggests.
"I'm having a bit of a difficulty getting my mind round the idea that a problem that has been caused or made worse by killing many many Arab Muslims in the Middle East is going to be made better by killing more with western weapons."
Shadow foreign secretary Douglas Alexander chose to direct his criticism at the Iraqi prime minister who he urged to act.
He said: "With the militant threat marching towards Baghdad, he must now act to deliver a strong Iraqi military response alongside a united and determined political front."Suggest a correction