Tony Blair said he would support the US if military action was needed in Iraq, the then-US Secretary of State claimed in a memo written a year before the war began.
The dossier, written on 28 March 2002 by former US Secretary of State Colin Powell to President George W Bush, said: "On Iraq, Blair will be with us should military operations be necessary.
"He is convinced on two points: the threat is real; and success against Saddam will yield more regional success."
The document was obtained by The Mail on Sunday as part of a batch of emails on the private server of Democratic presidential candidate Hillary Clinton, which US courts forced her to reveal. It was written a week before Mr Blair's meeting with Mr Bush at his Crawford ranch in Texas.
A spokesperson for Mr Blair told the paper the memo's content was consistent with what he had previously said publicly.
Mr Blair, who served as prime minister between 1997 and 2007, has repeatedly denied rushing to war in Iraq.
During his appearance before the Chilcot inquiry in January 2010, he denied he had struck a secret deal with Mr Bush at Crawford to overthrow Saddam Hussein.
Mr Blair said the two men had agreed on the need to confront the Iraqi dictator, but insisted they did not get into "specifics".
"The position was not a covert position, it was an open position," he said.
"This isn’t about a lie or a conspiracy or a deceit or a deception. It’s a decision.
"What I was saying... was 'We are going to be with you in confronting and dealing with this threat'."
When pressed on what he thought Mr Bush took from the meeting, he said the President had realised Britain would support military action if the diplomatic route had been exhausted.
At the inquiry, Mr Blair said he "regretted deeply and profoundly the loss of life" during and after the 2003 conflict.
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