Tony Blair and George Bush may have agreed the need for regime change in Iraq in private discussions at the US president's ranch, the Iraq Inquiry has heard.
Sir Christopher Meyer, who was Britain's ambassador to the US between 1997 and 2003, said the April 2002 meeting in Crawford, Texas, appeared to be a major turning point.
He said Mr Blair's view on the need to remove Saddam Hussein from power "tightened" after the talks in 2002.
Sir Christopher said in evidence: "I took no part in any of the discussions and there was a large chunk of that time when no adviser was there. I know what the Cabinet Office says were the results of the meeting but to this day I am not entirely clear what degree of convergence was, if you like, signed in blood at the Crawford ranch."
He said the change in stance was evidenced in a speech given by the Prime Minister the following day.
"To the best of my knowledge, I might be wrong, this was the first time that Tony Blair had said in public 'regime change'," Sir Christopher said.
"What he was trying to do was to draw the lessons of 9/11 and apply them to the situation in Iraq, which led - I think not inadvertently but deliberately - to a conflation of the threat posed by Osama bin Laden and Saddam Hussein.
"When I heard that speech, I thought that this represents a tightening of the UK/US alliance and a degree of convergence on the danger Saddam Hussein presented."
During Wednesday's session, it emerged that the Government was warned just days before the invasion of Iraq that Saddam Hussein may be unable to use his chemical weapons. Senior Foreign Office officials said intelligence was received in March 2003 suggesting Saddam had "disassembled" his chemical weapons and had not given the order to reassemble them.
The officials said ministers had been repeatedly warned the intelligence on Iraq's supposed weapons of mass destruction (WMD) was "patchy", even though Mr Blair told Parliament that they represented a "clear and present" threat. The inquiry is expected to last into 2011.
