He served as the Chancellor of the Exchequer in the Blair government when the decision to go to war as part of the US-led coalition was made in 2003.
The US kept quiet about the secret intelligence report, Mr Brown said according to report by the Mirror. "Britain would not have invaded Iraq had we known about it," he was quoted as saying by the paper. The report also undermined Britain's so-called "dodgy dossier" which claimed that Iraq could launch a biological attack on the UK within 45 minutes, the paper reported.
"We were all misled on the existence of WMDs," Mr Brown writes in his new book, 'My Life, Our Times', which is being published on Tuesday.
"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."
According to Mr Brown, he was assured by then MI6 chief Sir Richard Dearlove that Iraq had WMDs. "I was told they knew where the weapons were housed. I remember thinking it was almost as if they could give me the street name and number where they were," he writes.
The document revealed 90 per cent of US-held information on Iraq's WMDs was based on "imprecise intelligence" and relied on "judgment rather than hard evidence".
Mr Brown says he had no idea that decision makers in America were already aware that the evidence on the existence of WMDs was weak, even negligible and in key areas non-existent. "It is astonishing that none of us in the British Government ever saw this American report.
"As we were later to discover, the intelligence had not established beyond doubt either that Saddam had continued to produce chemical and biological weapons or that efforts to develop nuclear weapons continued," he writes.
As the Chancellor of Exchequer, Brown says his only official role was to find funds for the war. The US-led coalition toppled Saddam Hussain's government in Iraq in 2003 invasion.