Tony Blair and George W Bush decided to invade
The book by Philippe Sands says the two leaders discussed going to war regardless of any United Nations view.
The book quotes a note from a meeting in January 2003. The war did not begin until March after a vote in Parliament.
'Disarm Saddam'
The revelations come in an updated edition of Mr Sands' book Lawless World, which caused controversy when it was first published in February 2005.
The government has always insisted military action was used as a last resort against Saddam Hussein's regime.
Blair told MPs on 25 February 2003: "Even now, we are prepared to go the extra step to achieve disarmament peacefully."
But the new book centres on a two hour meeting between Mr Bush and Blair at the White House three weeks earlier, on 31 January.
Professor Sands, a QC and professor of international law at University College London, says the two-hour meeting was also attended by six advisers.
The book quotes from a note it says was prepared by one of the participants.
Surveillance aircraft
According to the note, Bush said the military campaign was pencilled in for March. Blair is quoted as saying he was "solidly with the president and ready to do whatever it took to disarm Saddam".
The book claims Mr Blair only wanted a second UN Security Council resolution because it would make it easier politically to deal with Saddam.
And it says Bush, told Blair the
If the Iraqis Saddam fired on them, the would be in breach of UN resolutions, he suggested.
Bush is also quoted saying it was possible an Iraqi figure would defect and be able to give a "public presentation" of weapons of mass destruction.
The note said Bush thought there was also "a small possibility that Saddam would be assassinated".
The book also claims the president "thought it unlikely that there would be internecine warfare between the different religious and ethnic groups".
'Will not re-litigate'
It also claims the note reveals the two leaders discussed a number of options.
A
A spokeswoman said the prime minister only committed
The decision to resort to military action into
The spokeswoman said No 10 did not comment on conversations between the prime minister and other leaders.
In the
Jones said the White House "was not going to re-litigate how the nation went to war".
Acting Liberal Democrats leader Sir Menzies Campbell said Blair had "a lot of explaining to do" and a full inquiry into the
"It would appear that the diplomatic efforts in
Sir Menzies said the suggestion that the
PHOTO CAPTION
Tony Blair and George W Bush. (BBC)
BBC