The UN's former chief weapons inspector Hans Blix has said it is his "firm view" that the Iraq war was illegal.
Dr Blix told the Iraq inquiry the UK had sought to go down the "UN route" to deal with Saddam Hussein but failed.
Ex-Attorney General Lord Goldsmith, who advised the war was lawful on the basis of existing UN resolutions, "wriggled about" in his arguments, he suggested.
Dr Blix said his team of inspectors had visited 500 sites but found no evidence of weapons of mass destruction.
As head of the UN's Monitoring, Verification and Inspection Commission (UNMOVIC) between 1999 and 2003, Dr Blix was a key figure in the run-up to the March 2003 invasion as he sought to determine the extent of Saddam's weapons programme.
'No smoking gun'
Asked about the inspections he oversaw between November 2002 and 18 March 2003 - when his team was forced to pull out of Iraq on the eve of the war - he said he was "looking for smoking guns" but did not find any.
While his team discovered prohibited items such as missiles beyond the permitted range, missile engines and a stash of undeclared documents, he said these were "fragments" and not "very important" in the bigger picture.
"We carried out about six inspections per day over a long period of time.
"All in all, we carried out about 700 inspections at different 500 sites and, in no case, did we find any weapons of mass destruction."
Although Iraq failed to comply with some of its disarmament obligations, he added it "was very hard for them to declare any weapons when they did not have any".
He criticised decisions that led to the war, saying existing UN resolutions on Iraq did not contain the authority needed, contrary to the case put by the UK government.
"Eventually they had to come with, I think, a very constrained legal explanation," he said. "You see how Lord Goldsmith wriggled about and how he, himself, very much doubted it was adequate."
Lord Goldsmith has acknowledged his views on the necessity of a further UN resolution mandating military action changed in the months before the invasion and that the concluded military action was justified on the basis of Iraq breaching disarmament obligations dating back to 1991.
But Dr Blix said most international lawyers believed these arguments would not stand up at an international tribunal.
"Some people maintain that Iraq was legal. I am of the firm view that it was an illegal war. There can be cases where it is doubtful, maybe it was permissible to go to war, but Iraq was, in my view, not one of those."
He said he agreed with France and Russia, who argued that further UN authorisation was needed for military action.
"It was clear that a second resolution was required," he said.
In the run-up to war, he said the US government was "high on" the idea of pre-emptive military action as a solution to international crises.
"They thought they could get away with it and therefore it was desirable to do so."
While he believed Iraq "unilaterally" destroyed its weapons of mass destruction after the 1991 Gulf War, Dr Blix said he never "excluded" the prospect that it had begun to revive some form of chemical and biological capabilities.
In September 2002, he said he told Tony Blair privately that he believed Iraq "retained" some WMD, noting CIA reports that Iraq may hold some anthrax.
However, he said he began to become suspicious of US intelligence on Iraq following claims in late 2002 that Iraq had purchased raw uranium from Niger, which he always said he thought was flawed.
Since the war, Dr Blix has accused the UK and US of "over-interpreting" intelligence on weapons to bolster the case for war but he said the government's controversial September 2002 dossier on Iraqi weapons seemed "plausible" at the time.
He stressed that Tony Blair never put any "pressure" on him over his search for weapons in Iraq and did not question that the prime minister and President Bush believed in "good faith" that Iraq was a serious threat.
"I certainly felt that he [Tony Blair] was absolutely sincere in his belief.
"What I question was the good judgement, particularly of President Bush but also in Tony Blair's judgement."
Critics of the war believe that had inspectors been allowed to continue their work they would have proved beyond doubt that Iraq did not have active weapons of mass destruction capability - as was discovered after the invasion.
Dr Blix said the military momentum towards the invasion - which he said was "almost unstoppable" by early March - did not "permit" more inspections and the UK was a "prisoner on this train".
If he had been able to conduct more inspections, he said he believed they would have begun to "undermine" US-UK intelligence on Iraq's alleged weapons and made the basis for the invasion harder.
The US and UK have always maintained that Saddam Hussein failed to co-operate fully with the inspections process and was continuing to breach UN disarmament resolutions dating back to 1991.
In his evidence in January, former foreign secretary Jack Straw said the regime had only started complying in the final period before the invasion "because a very large military force was at their gates".
The inquiry, headed by Sir John Chilcot, is coming towards the end of its public hearings, with a report expected to be published around the end of the year.