Hans Blix singled out Prime Minister Tony Blair for particular criticism, saying he had behaved like a "vendor with some merchandise" in promoting government support for the US-led invasion and occupation.
In an interview with BBC television on Sunday, the retired senior UN official said Blair had manipulated intelligence in a successful attempt to peddle an unpopular war.
His "intention was to dramatise it, just as the vendors of some merchandise are trying to increase or exaggerate the importance of what they have".
Blix was referring to the prominence Blair gave in a September 2002 intelligence dossier to a claim that Saddam Hussein could deploy chemical and biological weapons within 45 minutes.
The dossier formed part of Blair's efforts to persuade a sceptical British public that pre-emptive military action was required to rid Iraq of weapons of mass destruction.
[Blair's] "intention was to dramatise it, just as the vendors of some merchandise are trying to increase or exaggerate the importance of what they have"
former chief UN weapons inspector
Blix's comments coincided with an article in Britain's Independent on Sunday newspaper which claimed the source for the 45-minute claim had actually left Iraq some years earlier.
The newspaper even claimed the intelligence officer had only obtained the 45-minute information second hand.
Blix added when the UN inspection team withdrew at the end of 1998, intelligence communities in the West "lost an important source" of information.
"They were relying upon defectors and much of what they got there was wrong."
However, the former Swedish diplomat concluded neither Blair nor US President George Bush acted in bad faith, but blamed "information management".