Gordon Brown has claimed the UK was "misled" over Saddam Hussein's weapons of mass destruction and the Iraq War was unjustified.
The former British Prime Minister, who was Chancellor when the decision to go to war in 2003 was made, has revealed top-secret US intelligence casting serious doubt over the dictator's destructive capabilities was not shared with Britain.
He claimed only after leaving office did he become aware of "crucial" papers held by the US Department of Defence and believes the course of history could have been different had the information been shared.
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."
He added some form of international action was appropriate due to Saddam's continued failure to comply with UN resolutions.
The UK was part of the coalition led by the US which invaded Iraq after American president George W Bush and Tony Blair accused the dictator of possessing weapons of mass destruction and having links to terrorists.
As chancellor, Mr Brown said his only official role was to find funds for the war.
After an inquiry lasting seven years, the Chilcot Report found the former Iraqi dictator posed ''no imminent threat'' at the time of the invasion of his country in 2003, and the war was unleashed on the basis of ''flawed'' intelligence.
Mr Brown said British intelligence which he and Mr Blair saw in 2002 suggested a capability, if not a production programme, of weapons of mass destruction.
However, the top-secret US report is said to have conceded that knowledge of the Iraqi nuclear weapons programme was based largely - perhaps 90% - on analysis of imprecise intelligence.
In his latest book My Life, Our Times - due for release on Tuesday - Mr Brown claims the paper suggested previous assessments 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 Rumsfeld's confidential paper, President 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".
It is not known who - if anyone - in the US administration had seen the dossier.