Gordon Brown has voiced his sadness at the huge loss of life in the conflict in Iraq.
Making his long-awaited appearance before the Chilcot Inquiry, the Prime Minister expressed "regrets" at failures to plan properly for the aftermath of the invasion.
However he strongly defended the decision to go to war, insisting that the international community had to deal with the threat posed by Saddam Hussein.
"Any loss of life is something that makes us very sad indeed," he said. "Nobody wants to go to war, nobody wants to see innocent people die, nobody wants to see their forces put at risk of their lives.
"Nobody would want to make this decision except in the gravest of circumstances where we were sure that we were doing the right thing.I think it was the right decision and made for the right reasons."
Mr Brown acknowledged that there "important lessons" to be learned from the way the country descended into chaos following the invasion.
"It was one of my regrets that I wasn't able to be more successful in pushing the Americans on this issue - that the planning for reconstruction was essential, just the same as planning for the war," he said.
However Mr Brown, who was Chancellor of the Exchequer at the time of the invasion in March 2003, insisted he had provided the funding needed for the conflict. He said: "I said immediately to the Prime Minister that the military options that were under discussion, there should be no sense that there was a financial restraint that prevented us doing what was best for the military."
"I told him that I would not - and this was right at the beginning - I would not try to rule out any military option on the grounds of cost, quite the opposite."
Mr Brown also insisted that he had been kept fully informed about developments in the run-up to the conflict, and had been given five briefings by the intelligence agencies.