Opening his evidence to the Chilcot Inquiry, Gordon Brown expressed his sadness for the loss of life in the conflict.
But he insisted that it had been necessary for the international community to confront Saddam Hussein.
"I think this is the gravest decision to go to war," he said. "It was the right decision and it was for the right reasons."
Mr Brown paid tribute to the "sacrifice" of the British troops who had lost their lives in the conflict while acknowledging the scale of the civilian losses.
"Any loss of life is something that makes us very sad indeed," he said.
Unlike Tony Blair when he appeared in January, Mr Brown entered the Queen Elizabeth II Conference Centre, where the inquiry is being held, by the front entrance.
Only a small knot of protesters had gathered to mark his arrival.
Mr Brown said there were three areas he particularly wanted to raise with the inquiry panel.
He said it was vital to have "proper structures of decision-making" in Government for fighting wars, to manage conflicts better and to improve how states work together in conflicts.
Mr Brown said: "There will be interventions in the future, and international co-operation has got to be far greater than it was.
"Global problems require global solutions, and I would particularly draw attention to the importance in all this of the strongest possible relationship between Europe and America, something that I am determined to build up and continue to make stronger in the future."
Mr Brown told the inquiry that terrorists and "rogue states" were the "two risks to the post-Cold War world" and had to be tackled.
He said he was involved in Cabinet discussions before the invasion and was convinced that Iraq posed a threat.
"I met the intelligence services on a number of occasions during the course of 2002 and early 2003," he said.
"I was given information by the intelligence services which led me to believe that Iraq was a threat and had to be dealt with by the actions of the international community."
But he told the panel that he had wanted to secure international agreement to avoid war.
"What we wanted was a diplomatic route to succeed," he said. "Right up to the last minute, right up to the last weekend, I think many of us were hopeful that the diplomatic route would succeed."
Mr Brown said he told Mr Blair in mid-2002 that money would be no object if Britain launched military action against Saddam Hussein.
The former chancellor said he understood that some options for a war were more expensive than others, but pledged to fund whichever option was "right for our country".
Mr Brown said he was involved in discussions with then-defence secretary Geoff Hoon about a possible conflict with Iraq from June 2002.
He told the inquiry: "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."
He added: "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 insisted that throughout the build-up to war, the Cabinet had been kept fully informed of developments.
"I cannot see the argument that says the Cabinet were not informed. We were informed fully about the process of the negotiations," he said.
He said that throughout the discussions, he had been adamant that military action should only be a last resort.
"I was very clear that we had to exhaust all the diplomatic avenues before we could conclude that it was inevitable or impossible to avoid a decision about war," he said.
However, he said that he did not see private correspondence between Mr Blair and US President George Bush.
"I would not expect to see private letters between Mr Blair and the president," he said.
Mr Brown conceded that he had "regrets" over not being "more successful" in convincing America of the need for better post-conflict planning.
"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.
He said there had been a Cabinet meeting in March 2003 that discussed "reconstruction issues".
Mr Brown told the inquiry that he had offered to "prepare a paper" to send to the United States on the issues that would have to be faced in post-war Iraq.
But he spoke of his frustration that dealing with the problems of the war-torn country were not dealt with "more quickly" after the initial invasion.
"I wish it had been possible to follow it through much more quickly in the aftermath," he said.
Mr Brown admitted that one lesson of the war was that spy chiefs had to be "more sure" about the nature of the intelligence they receive.
Claims that Saddam had weapons of mass destruction (WMD) that he could deploy within 45 minutes were discredited when thorough searches after the invasion failed to discover any such devices.
Mr Brown said he received five "very full" intelligence briefings about Iraq's alleged WMD in the lead-up to war, on March 4 2002, December 9, December 13, February 6 2003 and February 24.
He said: "The information I was given was that there was evidence that was known to many countries, not just our country, about the weaponry that the Iraqi government held.
"And, of course, at that time there was a greater certainty amongst the intelligence community that this weaponry was there.
"I think we have learned that intelligence can give us insights into what is happening, but we have got to be more sure, as people have recognised, about the nature of the intelligence we were receiving from certain people."
The inquiry heard that Mr Brown was not shown an "options paper" drawn up by the Cabinet Office in March 2002 to outline various choices for dealing with Iraq, from continuing sanctions to launching regime change.
The Prime Minister said: "I don't recall seeing that paper. My main involvement in looking at the options started from June."
He added: "I don't think I needed to see every paper."
Previously, the inquiry heard evidence from former No 10 communications director Alastair Campbell that Mr Blair had pledged to America that Britain would "be there" in the event of an invasion - several months before the decision was made to go to war.
Panel member Sir Roderic Lyne asked a "yes or no" question about whether Mr Brown was aware of letters Mr Blair had sent to US president George Bush indicating Britain's support.
Mr Brown responded: "I had regular conversations with Tony Blair and we talked about those issues but I do not have copies of those letters and I don't know the exact conversation - and he wouldn't expect me to."
Mr Brown acknowledged that he had not been present at a number of key meetings held by Mr Blair in the build-up to the invasion, but insisted that he was kept fully informed.
"I didn't at any point (feel) that I lacked the information that was necessary," he said.
"My role in this was to second guess military decisions or options. My role in this was not to interfere in what were very important diplomatic negotiations.
"My role in this was, first of all as chancellor of the exchequer, was to make sure that the funding was there for what we had to do."
Mr Brown insisted he and other senior Cabinet members were kept fully informed about developments in the months before the invasion.
He said: "There is no sense in which I felt that I had inadequate information.
"Obviously, the intelligence information has had to be re-assessed as a result of what we have now learned.
"But there was no sense in which we were denied information that was necessary for us making a decision."
The Independent
