Tony Blair has been heavily criticised over his rejection of claims that the 2003 invasion was to blame for the violent insurgency in Iraq.
Writing in an essay published on his website, the former prime minister said it was a "bizarre" reading of the situation to assert that the US-British invasion of Iraq had enabled the growth of Sunni jihadist organisations such as the Islamic State in Iraq and the Levant (Isis).
Thousands have fled the advance of fighters from the al-Qaida-linked Isis, who have seized large areas including Mosul and Tikrit.
Mr Blair said that the current crisis was caused by the Arab Spring, which he said would have swept Saddam from power and caused chaos without his decision to intervene.
"We have to liberate ourselves from the notion that 'we' have caused this. We haven't. We can argue as to whether our policies at points have helped or not: and whether action or inaction is the best policy. But the fundamental cause of the crisis lies within the region not outside it.
"We have to put aside the differences of the past and act now to save the future," says Blair, adding that force may be necessary. "Where the extremists are fighting, they have to be countered hard, with force."
Lord Prescott, the former deputy prime minister, criticised Mr Blair's comments. "Put on a white sheet and a red cross and we are back to the Crusades," he said. "It is all about religion. In these countries it has gone on for a thousand years."
Sir Christopher Meyer, Britain's ambassador to the US from 1997 to 2003, said Mr Blair's campaign was "perhaps the most significant reason" for the current sectarian violence in 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 said.
Writing in the Daily Telegraph, Boris Johnson said Mr Blair was undermining the case for "serious and effective intervention" and described the invasion as a "tragic mistake".
"Blair's argument (if that is the word for his chain of bonkers assertions) is that we were right in 2003, and that we would be right to intervene again," he wrote.
John Baron, a Conservative MP who led opposition to the strikes, said: "There is no doubt we went to war in Iraq on a false premise and made grave errors in the immediate aftermath in leaving a power vacuum. A large part of the troubles today can be traced back to that period."
Others, including journalist Owen Jones and Diane Abbott, the Labour MP for Hackney North and Stoke Newington, commented on Twitter.