"If I am right that somewhere within the American system the truth about Iraq's lack of weapons was known, then we were not just misinformed but misled on the critical issue of WMDs.
"Given that Iraq had no usable chemical, biological or nuclear weapons that it could deploy and was not about to attack the coalition, then two tests of a just war were not met: war could not be justified as a last resort and invasion cannot now be seen as a proportionate response," he added.
Britain joined the US, led by then-president George W. Bush, in invading Iraq following allegations that Saddam Hussein was storing WMDs.
The UK government compiled two dossiers, one of which was dubbed the 'Dodgy Dossier' by the British press, in which military action in Iraq was justified to the public.
Among the claims made about Iraq's Ba'athist regime was that it could deploy biological weapons within 45 minutes of an order.
The intelligence seen by Brown after his resignation suggests that as much as 90 percent of the intelligence on Iraq's weapons capabilities relied on assumptions.
Brown admits this in his in latest book My Life, Our Times, which is due for release next week.
"I was told they knew where the weapons were housed. I remember thinking at the time that it was almost as if they could give me the street name and number where they were located," the former prime minister recounts.
"It is astonishing that none of us in the British government ever saw this American report."
