Iraqis would have rebelled against Saddam Hussein if there had been no invasion and it would have been "a lot worse than Syria", Tony Blair has said.
Iraqis previously "rose up in large numbers and were killed in very large numbers", the former UK PM said.
Mr Blair also warned the West could pay a high price if it decided not to intervene in Syria.
He spoke to the BBC ahead of the 10th anniversary of the invasion, led by the US in coalition with the UK and others.
The invasion, which started the Iraq War, began on 20 March 2003.
The UK lost 179 servicemen and women, of which 136 were killed in action, before the last British troops were withdrawn in April 2009.
Conservative estimates put the number of Iraqis killed in the invasion and ensuing sectarian violence at about 100,000.
Mr Blair told the BBC's Norman Smith that, had troops not invaded Iraq, "without any doubt at all there would have been a move to topple him".
"I certainly think that if Saddam had still been in power, it's true there would have been, probably, an uprising amongst his people," he said.
"But I think it would look a lot more like Syria and probably a lot worse than Syria."
Thousands of Syrians have lost their lives in the battle between forces loyal to President Bashar al-Assad and those opposed to his rule.
Mr Blair said he regretted how difficult Iraq had been and the loss of life, but he did not regret the decision to oust Saddam.
Mr Blair said Saddam Hussein had used chemical weapons on his own people "and that's why, when people say to me, 'do you regret removing him', my answer is 'no - how can you regret removing somebody who was a monster, who created enormous carnage - not just amongst his own people but amongst the people of the region'".
Mr Blair said he believed "very sincerely and deeply" that the former Iraqi leader was "a security threat" who had to be dealt with.
"And if you look at what's happening in the Arab Spring today and you examine what's happening in Syria - just reflect on what Bashar al-Assad, who is a 20th as bad as Saddam, is doing to his people today and the number of lives already lost.
"Just ask yourself the question: 'What would be happening now in Iraq if he had been left in power?'"
Speaking about the Syrian conflict, the former prime minister warned there was "no end in sight" and the cost of not intervening would be "very high".
"If things continue as they are in Syria today, within a few months, proportionate to the size of the population, more people will have died in Syria than in the whole of the conflict since 2003 in Iraq," he added.
In 2010, at the Chilcot Inquiry into the Iraq War, Mr Blair was questioned about the controversial claim in a September 2002 dossier that Iraq could deploy weapons of mass destruction at 45 minutes' notice.
He told the inquiry that it "assumed a vastly greater significance" afterwards than it did at the time.
He has insisted that, on the basis of the intelligence available at the time, it was "beyond doubt" Iraq was continuing to develop its weapons capability.
Last month, Mr Blair's deputy prime minister at the time of the 2003 invasion, Lord Prescott, said it "cannot be justified as an intervention".
He said he could not "just disown it" but he now thought the war was wrong.