Tony Blair has denied striking a "covert" deal with George Bush to invade Iraq at a private meeting in 2002 at the US president's ranch, BBC reported.
Mr Blair said he had been "open" about what had been discussed - that Saddam Hussein had to be dealt with and "the method of doing that is open".
He said he had told the US president: "We have to deal with his WMD and if that means regime change so be it."
The former prime minister is being questioned by the Iraq inquiry.
Earlier witnesses have suggested that Mr Blair told Mr Bush at their April 2002 meeting at the ranch in Crawford, Texas, that the UK would join the Americans in a war with Iraq.
But Mr Blair said: "What I was saying - I was not saying this privately incidentally, I was saying it in public - was 'we are going to be with you in confronting and dealing with this threat'.
"The one thing I was not doing was dissembling in that position. How we proceed in this is a matter that was open. The position was not a covert position, it was an open position."
Pressed on what he thought Mr Bush took from the meeting, he went further, saying: "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 also denied he would have supported the invasion of Iraq even if he had thought Saddam Hussein did not possess weapons of mass destruction (WMD), as he appeared to suggest last year in a BBC interview.
What he had been trying to say, he explained to the inquiry, was that "you would not describe the nature of the threat in the same way if you knew then what you knew now, that the intelligence on WMD had been shown to be wrong".
He said his position had not changed, despite what reports of the interview had suggested.
Throughout the morning session, Mr Blair was at pains to point out that he believed weapons of mass destruction and regime change could not be treated as separate issues but were "conjoined".
He said "brutal and oppressive" regimes with WMD were a "bigger threat" than a benign states with WMD.
He also stressed the British and American attitude towards the threat posed by Saddam Hussein "changed dramatically" after the terror attacks on 11 September 2001, saying: "I never regarded 11 September as an attack on America, I regarded it as an attack on us."
The former PM said that the policy up to that point was one of "containment" but it was transformed into the view that Saddam had to be be "dealt with".
"That completely changed our assessment of where the risks for security lay," he said.
"And, just so we get this absolutely clear, this was not an American position - this was my position and the British position."
At the meeting with Mr Bush in April 2002, the US president had "expressed his view that if we were not prepared to act in a really strong way we would run the risk of sending a disastrous signal to the world", said Mr Blair.
Inquiry chairman Sir John Chilcot began the six hour question session by stressing that Mr Blair was not "on trial" but said he could be recalled to give further evidence if necessary.
Families of some of the 179 British service personnel killed in Iraq are watching the evidence with others said to be joining the 200 or so anti-war protestors outside.
Rose Gentle's son, Gordon, was killed by a roadside bomb in Basra in 2004, while serving with the Royal Highland Fusiliers.
She said the families of the dead wanted closure and for Mr Blair to explain "in depth" to the families and and the public "why he went in" as she said he had never done that before.