Gordon Brown said today it had been a "mistake" to fail to prepare adequately for the reconstruction of Iraq following the removal of Saddam Hussein.
But the Prime Minister insisted the Government had to take the action it did, in invading the country, because of the Iraqi dictator's refusal to comply with United Nations demands.
His comments, at his monthly Downing Street press conference, come ahead of his appearance before the Chilcot Inquiry.
It was announced last week that he would now answer the committee's questions before the general election.
Challenged today about the legality of the invasion, Mr Brown said he would address such issues when he appears before the inquiry.
But he went on: "I think the mistake in the war was not to do the reconstruction and plan it in the way that was necessary so that Iraq could recover quickly after Saddam Hussein fell.
"As far as the war is concerned, I have always said that the UN resolution that said to Saddam Hussein for over 10 years that he had to take action to deal with the threat he was posing to other countries was a very decisive part of the reasons why we had to take the actions we did."
Mr Brown told reporters: "I stand by all the actions I have taken and I welcome the chance to explain not only the circumstances in which our Government made the decisions it did, but also the circumstances in which we brought our troops home from Iraq.
"I welcome the chance to appear before the Iraq Inquiry. I had offered to appear on any date - the Inquiry thought it better after the election.
"I said I was happy to appear on any date and then they offered some other dates."