Tony Blair turned down a chance to keep British troops out of fighting in Iraq, according to a prominent US journalist.
Woodward interviewed Mr Bush and administration officials for his book
In a new book, Bob Woodward says George W Bush began planning the Iraq war in December 2001.
But he gave Mr Blair time to seek a second UN resolution, as he feared the UK government could fall on the issue.
Mr Woodward, along with Carl Bernstein, uncovered the Watergate scandal which ultimately led to Republican President Richard Nixon's resignation.
In 'Plan of Attack', Mr Woodward says the decision to go to war had been taken in January 2003 when the UN route was still being pursued.
But it is Mr Woodward's claim that Mr Blair was given a get-out clause on military action - that would not have had political consequences for Anglo-American relations - that is likely to of be of most interest in Britain.
Speaking on BBC2's Newsnight programme the legendary reporter said: "Blair was very direct in saying: 'Look I made a commitment.'
Interview with Bush
"[The president] offered this extraordinary opportunity to essentially remove the UK from a combat role in Iraq and the prime minister said: 'Absolutely not. I have made the commitment and I am with you to the end'."
The suggestion that Mr Bush began secret planning for the war in Iraq in December 2001 is likely to give fuel to critics who have accused the president of being too eager to go to war with Saddam Hussein.
Mr Woodward interviewed Mr Bush and his administration officials for the book.
He reportedly told Mr Woodward he would co-operate because he wanted the story of how the US had gone to war in Iraq to be told.
The journalist says Mr Bush met repeatedly with his war cabinet in late 2001 - three months after the 11 September attacks on the US - to plan the US attack on Iraq.
But his book suggests the US president did not brief everyone on his national security team.
And Mr Bush apparently told the journalist that if the news had leaked, it would have caused "enormous international angst and domestic speculation".
The book puts the blame for mistaken intelligence on weapons of mass destruction on the CIA director George Tenet, who reportedly assured the president the weapons existed.
Mr Woodward also says vice-president Dick Cheney was a
"steamrolling force" behind war preparations and that the relationship between Mr Cheney and Secretary of State Colin Powell became severely strained.
Cat out of the bag?
Just before war began in March 2003, US Defense Secretary Donald Rumsfeld caused political embarrassment to the UK Government saying Britain's role in any military action was "unclear".
He said it was because of Mr Blair's difficulties in convincing Parliament of the need for military action.
Asked if he meant the US would go to war without its closest ally, he added: "That is an issue that the president will be addressing in the days ahead, one would assume."
At the time Mr Blair's spokesman insisted British troops would play a "significant" role if the war seemed necessary .
But Mr Rumsfeld's remarks prompted Labour MP Graham Allen to say: "The cat is out of the bag. They can do it without us and give Tony Blair the chance to get out of the hole if he wishes."