In a book, excerpts of which Britain's The Guardian newspaper published on Saturday, Blix says that in the run-up to war, the British prime minister and envoys of the US president seemed convinced by the information from their intelligence agencies.
Blix, who said he came under intense US pressure to accept such intelligence as fact and was vilified for refusing, said he personally believed Iraqi President Saddam Hussein still had hidden illegal weapons, but had told Blair he needed proof.
"I added that it would prove paradoxical and absurd if 250,000 troops were to invade Iraq and find very little," he wrote about their meeting on 20 February 2003.
"Blair responded that the intelligence was clear that Saddam had reconstituted his weapons of mass destruction programme. Blair clearly relied on the intelligence and was convinced."
Blix wrote that Western intelligence claims shared with his inspectors about, for example, mobile laboratories to make biological agents had proved embarrassing and added: "I am not aware of any other intelligence 'shared' with us that has been substantiated by credible evidence.
"Perhaps Blair and Bush, both religious men, felt strengthened in their political determination by the feeling they were fighting evil, not only [arms] proliferation"
former chief UN arms inspector
"Perhaps Blair and Bush, both religious men, felt strengthened in their political determination by the feeling they were fighting evil, not only [arms] proliferation," he wrote.
In the new book Disarming Iraq - The Search for Weapons of Mass Destruction, Blix said French intelligence services had also been convinced WMDs remained in Iraq, but that President Jacques Chirac - as staunchly opposed to war as Bush and Blair were in favour - was more sceptical.
"The intelligence services sometimes 'intoxicate each other'," he said, citing Chirac.
Blix described an increasingly frantic round of diplomatic activity as the troop build-up in Kuwait gathered pace and the arms inspectors scouring Iraq came up empty-handed.
Nearly a year after the invasion and Saddam's overthrow, US-led forces have not found any illegal weapons.
Blair and Bush have both seen their popularity plummet over the unpopular war and its bloody aftermath.
On Friday Blair raised the prospect of a rethink of international law and urged the United Nations to legalise pre-emptive strikes by foreign forces against so-called rogue states.