In an interview published on Thursday, Attorney-General Lord Peter Goldsmith told the Daily Telegraph newspaper that "I stand by my conclusion that military action was lawful".
This is the first time Goldsmith has broken his silence on what was a major issue of controversy in the British general election earlier this month.
"That was a judgement I had to reach. I reached it and I stand by it," Goldsmith said.
"And I want to reject the suggestions that I was leant on, or that this somehow was not genuinely my view - these are fantasies and they need to be seen as such."
Three days before the US-led, British-backed invasion of Iraq began in March 2003, Goldsmith told parliament it was permitted under international law even without a final United Nations resolution to authorise it.
However, opposition politicians charged that Goldsmith had earlier told Prime Minister Tony Blair and other ministers that he had severe doubts about the conflict's legality, and was pressured into changing his view.
Blair and Goldsmith consistently refused to release this confidential legal advice to ministers, but the document was released just before the 5 May election after extracts of it were leaked to the press.
In the 7 March 2003 memo, Goldsmith seemed to doubt whether Iraq could legitimately be attacked without one final resolution ordering Saddam Hussein to fully cooperate with UN inspectors and abandon his suspected development of weapons of mass destruction.
"And I want to reject the suggestions that I was leant on, or that this somehow was not genuinely my view - these are fantasies and they need to be seen as such"
Lord Peter Goldsmith,
"In these circumstances, I remain of the opinion that the safest legal course would be to secure the adoption of a further resolution to authorise the use of force," Goldsmith wrote.
He went on to say it would be "surprising" if there were no legal challenge from opponents of the war, adding: "We cannot be certain that they would not succeed."
Insisting the second, less ambiguous ruling was his own thinking, Goldsmith said he would have had no qualms about bringing Blair bad news if necessary.
"I would not have hesitated to give negative advice if that had been my conclusion. I have been a practising lawyer for 30 years," he told the newspaper.
"I have always operated on the basis that I do not tell clients just what they would like to hear."
Goldsmith, who said he had not talked about the issue previously because the 7 March memo had originally been confidential, argued his two pieces of advice were not contradictory.
The first memo weighed up the possibilities, but by the time he spoke to parliament it seemed clear there would be no new UN resolution, and he was being pushed for a "yes or no" answer on the war's legality, Goldsmith said.
"I had to reach a conclusion, having weighed up all the arguments. It could not be slightly lawful: it either was or it wasn't. My conclusion was that it was lawful."