Tony Blair said he believes it still would have been right to have invaded Iraq even if it was known then that Saddam Hussein did not have weapons of mass destruction.
The former Prime Minister - who is due to give evidence in the New Year to the Chilcot inquiry into the war - said other arguments would have been needed to justify the military action in 2003.
But in an interview to be broadcast tomorrow on BBC1's Fern Britton Meets ... Tony Blair, he said the threat posed by Saddam to the wider region meant it was right to remove him from power.
"I would still have thought it right to remove him. Obviously you would have had to use and deploy different arguments, about the nature of the threat," he said.
"I can't really think we'd be better with him and his two sons still in charge but it's incredibly difficult.
"I sympathise with the people who were against it for perfectly good reasons and are against it now, but for me, in the end, I had to take the decision."
He added: "It was the notion of him as a threat to the region, of which the development of WMD was obviously one, and because you'd had 12 years of United Nations to and fro on this subject, he used chemical weapons on his own people - so this was obviously the thing that was upper most in my mind."
Mr Blair acknowledged that there were families who blamed him for the deaths of loved ones in a conflict in which they believed Britain should never have been involved.
"That's the responsibility you carry. But you have got to carry it, I'm afraid... There is no point in going into a situation of conflict and not understanding there is going to be a price paid," he said.
"It's also important to understand that many of those who are in the armed forces, including those who have lost their loved ones in Afghanistan or in Iraq, they are very often proud of what their child has done and proud of the cause they fought in, so you've got to be.
"You know there are parents who feel very very deeply angry and resentful and believe that the war was not worth it."
Mr Blair, who was always reluctant to discuss his Christian faith while he was in office, said that while it did not affect his decision to go to war, it did help to sustain him through the conflict.
"I actually genuinely believe with a decision like this you've got to take it, you've got to work out what you think is right," he said.
"What your faith does is it sustains you through what is then a very difficult time as you try to implement what you think is right. What your faith can't do, I'm afraid, is tell you what is the right thing."
Mr Blair, who converted to Catholicism following his resignation as Prime Minister in 2007, said that he waited until he left office to do so because of the controversy it would have caused.
"There would have been endless hassle," he said.
"Maybe I should just have done it but, to be frank, you have got so much going on as Prime Minister and there are so many issues you are having to deal with, that you really wonder whether it's a great idea to put the whole Catholic versus the established church thing into it.
"I had enough controversy to deal with."