The revelations focus on a memo allegedly written by former US secretary of state Colin Powell on March 28, 2002 to then president George Bush a week before the US leader's meeting with Blair at his ranch in Crawford, Texas, AFP reportes.
"On Iraq, Blair will be with us should military operations be necessary," wrote Powell, in a document the Mail on Sunday published on its website.
"He is convinced on two points: the threat is real; and success against Saddam will yield more regional success," Powell said, referring to former Iraqi leader Saddam, who was eventually ousted in the 2003 US-led invasion.
The Mail Sunday said the memo and other sensitive documents were part of a batch of secret emails held on the private server of Democratic presidential candidate Hillary Clinton which US courts have forced her to reveal.
A separate quote from Powell assured Bush "the UK will follow our lead in the Middle East", while other statements suggest Blair's willingness to present "strategic, tactical and public affairs lines" to strengthen public support for the Iraq war.
Blair, who served as prime minister between 1997 and 2007, has repeatedly denied rushing to war. Under his leadership, Britain made the second biggest troop contribution to the Iraq invasion, and British forces were stationed in the country until 2011.
The decision to back the invasion is now deeply unpopular in Britain and has haunted Blair's Labor Party ever since.
His office was not immediately available for comment.