Gordon Brown has claimed that “crucial” US intelligence that cast doubt Iraq’s weapons of mass destruction was concealed for the UK in the build-up to the war.
The information, which he says he did not see until after leaving office, would have stopped him from supporting the decision to invade.
Mr Brown said: “When I consider the rush to war in March 2003 – especially in light of what we now know about the absence of weapons of mass destruction – I ask myself over and over whether I could have made more of a difference before that fateful decision was taken.
“We now know from classified American documents, that in the first days of September 2002 a report prepared by the US Joint Chiefs of Staff’s director for intelligence landed on the desk of the US Defence Secretary, Donald Rumsfeld.
“Commissioned by Rumsfeld to identify gaps in the US intelligence picture, it is now clear how forcibly this report challenged the official view.
“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 claims appear in Mr Brown’s memoir My Life, Our Times – published on Tuesday. He says the US Department of Defence document claimed the assessment of Saddam’s capability relied “heavily on analytic assumptions” rather than hard evidence and even refuted the country’s capability to create weapons of mass destruction.
He added: “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.
“It is astonishing that none of us in the British government ever saw this American report.”
One month after Mr Rumsfeld’s confidential paper, President George W Bush went on record for the first time with the assertion that Iraq “possesses and produces chemical and biological weapons” and was “seeking nuclear weapons”.
Last year, Sir John Chilcot’s 12-volume report into the war concluded that the intelligence reports that led the then Prime Minister Tony Blair to take the country to war had not established “beyond doubt” that they existed.
It also concluded Mr Blair was so convinced of the presence of the non-existent WMDs that he sent British troops into Iraq when diplomacy might still have resolved the crisis. But the secret intelligence reports he had been shown “did not justify” his certainty, Sir John concluded.
The issue of whether the then Prime Minister lied to Parliament to justify the UK’s involvement in the Iraq war has been a source of damaging controversy for more than 14 years.
