The prime minister knew Iraq had no weapons of mass destruction ready for use within 45 minutes, former foreign secretary Robin Cook has claimed.
He said that before the war started Mr Blair privately admitted that Saddam Hussein had no weapons posing a "real and present danger".
Mr Cook - who resigned as Leader of the Commons in protest at the conflict - makes his claims in a book based on his diaries, being serialised in the Sunday Times.
Downing Street has dismissed the allegations as "absurd".
But the Liberal Democrats and a former Labour minister have both predicted the publication of Mr Cook's diaries will spell another tough period for the prime minister.
Mr Cook said it became clear that the prime minister did not consider Iraq's weapons an imminent threat when they spoke two weeks before war started.
He added that Mr Blair had appeared prepared to go to war regardless of any progress made by UN weapons inspectors.
Mr Cook's new book is based on diaries kept during the tense period in the run-up to the conflict.
In extracts published on Sunday, Mr Cook also says the head of the Joint Intelligence Committee, John Scarlett, "assented" when he suggested Iraq had no WMD which could target large cities.
And he claims that a "large number of ministers" spoke up in cabinet against British involvement in the US-led military action.
It was the nearest thing to a "mutiny" he had seen since Mr Blair took office, said Mr Cook, who quit his cabinet post in March in protest at the prospect of military intervention.
A Downing Street spokesman said: "The idea that the prime minister ever said that Saddam Hussein didn't have weapons of mass destruction is absurd.
"His views have been consistent throughout, both publicly and privately, as his cabinet colleagues know.
"Robin Cook's views are well known and have been expressed many times before."