The US defence department knew that Saddam Hussein did not have weapons of mass destruction but kept Britain in the dark, according to an explosive new claim from Gordon Brown.

In an extraordinary allegation, the former prime minister states that a secret US intelligence report into Iraq’s military capabilities was never passed to Britain and could have changed the course of events. The revelation leads Brown to conclude that the “war could not be justified as a last resort and invasion cannot now be seen as a proportionate response”.

He adds that the evidence in question was never examined by the Chilcot inquiry into the Iraq war, which concluded that Britain chose to join the invasion before “peaceful options for disarmament” had been exhausted. Brown’s intervention will reopen the debate about Britain’s decision to join the US-led invasion of Iraq. Tony Blair used the assertion that Iraq had weapons of mass destruction to argue that Britain needed to join the military action.

Brown makes the claim in his new book, My Life, Our Times, published this week. He writes that there was a “rush to war” in March 2003, adding that he asks himself “over and over whether I could have made more of a difference before that fateful decision was taken”.

He said that as chancellor he had little more access to intelligence than other cabinet ministers, but was reassured by MI6 that evidence about WMDs was well-founded. However, having reviewed the evidence since leaving office, he writes that he now believes “we were all misled on the existence of WMDs”.

Brown points to a crucial set of papers from September 2002, commissioned by the then US defence secretary, Donald Rumsfeld, and held by the US defence department, which was leaked last year. According to Brown, it made clear that evidence of “the existence of WMDs was weak, even negligible and in key areas nonexistent”.

It is astonishing that none of us in the British government ever saw this American report

“It is astonishing that none of us in the British government ever saw this American report,” Brown writes. “It is now clear how forcibly this report challenged the official view: ‘We’ve struggled to estimate the unknown … We range from 0% to about 75% knowledge on various aspects of their [Iraq’s WMD] program’,” the report stated.

“It conceded that US knowledge of the Iraqi nuclear weapons programme was based largely – perhaps 90% of it – on analysis of imprecise intelligence. These assessments, the report said, relied ‘heavily on analytic assumptions and judgment rather than hard evidence. The evidentiary base is particularly sparse for Iraqi nuclear programs.’

“The Iraqis, it was reported, ‘lack the precursors for sustained nerve-agent production’, confirming that US intelligence could not identify any Iraqi sites producing the final chemical agent. And as for missiles and the Iraqis’ ability to target countries such as the UK with them, which was to be the subject of dramatic claims only a few weeks later, Rumsfeld was informed: ‘We doubt all processes are in place to produce longer-range missiles’.

“This highly confidential US evidence was a refutation not only of the claim that Iraq was producing WMDs but also of their current capability to do so.”

Brown states that had the evidence been shared, history could have been different. “I am convinced that if resolutions of the United Nations are approved unanimously and repeatedly they have to be upheld if we are to have a safe and stable world order,” he writes. “On this basis, Saddam Hussein’s continuing failure to comply with them justified international action against him.

“The question is whether it required war in March 2003. 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.”

The former prime minister also reveals that the moment he took office in 2007, he planned to pull British troops out of Iraq well before the Americans. He eventually withdrew troops in April 2009, while the US stayed until December 2011. “At this time I made another decision: not to use our future departure from Iraq as an occasion to draw a contrast with Tony or score points against him either,” he writes.