Former UK PM Tony Blair would support the US if military action was needed in Iraq, a top US official claimed in a memo written a year before the war.
The document, written in March 2002 by ex-US Secretary of State Colin Powell to President George W Bush, has been published by the Mail on Sunday.
It said: "Blair will be with us should military operations be necessary."
A spokesman for Mr Blair said the memo's content was consistent with what he had said publicly before.
The briefing note emerged after a court ruling in the US led to the publication of thousands of emails received by Hillary Clinton, who served as Secretary of State under President Obama.
BBC political correspondent Chris Mason said the document appeared to offer a revealing insight into how Mr Blair was perceived at the very highest level in Washington, in the year before the Iraq War.
The Mail on Sunday says the document was written a week before Mr Blair met Mr Bush at the former president's ranch in Crawford, Texas - and a year before MPs voted to support the invasion of Iraq.
Mr Powell writes that the then prime minister is convinced "the threat is real" and "success" against the former Iraqi President Saddam Hussein "will yield more regional success".
In reference to the so-called war on terror, Mr Powell goes on to praise what he sees as Blair's impressive public relations skills.
"He will present to you [George Bush] the public affairs lines that he believes will strengthen global support," the document says.
Another document included in the batch of emails published by the paper is a briefing for Mr Powell prepared by the US Embassy in London, shortly before the leaders met at the ranch in Texas.
'Policy of containment'
The memo, dated April 2002, includes an assessment of the effect on Mr Blair's domestic position if he backs US military action.
The document says: "A sizeable number of his [Blair's] MPs remain at present opposed to military action against Iraq... some would favor shifting from a policy of containment of Iraq if they had recent (and publicly usable) proof that Iraq is developing WMD/missiles... most seem to want some sort of UN endorsement for military action."
The reasons for going to war in Iraq - including the now discredited claim that Saddam Hussein had weapons of mass destruction which could be used within 45 minutes of an order being given - remain a source of controversy.
Mr Blair, who served as prime minister between 1997 and 2007, has previously denied rushing to war.
He told the Chilcot Inquiry into the war in 2010 he had been "open" about what he had told Mr Bush in private, that "we are going to be with you in confronting and dealing with this threat".
"I think what he took from that was exactly what he should have taken, which was if it came to military action because there was no way of dealing with this diplomatically, we would be with him," Mr Blair added at the time.
A spokeswoman for Mr Blair said: "This story is nothing new. The memo is consistent with what Mr Blair was saying publicly at the time and with Mr Blair's evidence to the Chilcot Inquiry."
The UK joined a US-led invasion of Iraq in March 2003 despite failing to secure a second UN resolution justifying the use of force.
SNP MP Alex Salmond said the memo was "extremely damaging" for Mr Blair.
"The illegal invasion of Iraq has been unequivocally proven as a fraud and a massive deception by Tony Blair and the then UK Labour government," he said.
Last month, amid delays to publication of the inquiry's report, relatives of UK soldiers killed in Iraq said they would begin legal action if it fails to publish by December.
Sir John Chilcot, the inquiry chairman who began his work in 2009, has said the so-called "Maxwellisation" process is over but has not announce a publication date.
He said last month he would write to Prime Minister David Cameron to set out a timetable for publication "as soon as I am able".