Sir Jeremy Greenstock, the former British ambassador to the United Nations, will give evidence to the official inquiry into the Iraq war today.
Greenstock was at the forefront of the failed attempts to win UN approval for the 2003 US-led invasion.
On Thursday, Sir Christopher Meyer, the former UK ambassador to the US, told the inquiry that Tony Blair and George Bush may have agreed on the need to oust Saddam Hussein up to a year before the invasion began.
He said that Blair's position on regime change "tightened" after critical talks at Bush's Texas ranch in March 2002.
He told the inquiry: "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 added that the "unforgiving nature" of the US military build-up left insufficient time for UN weapons inspectors to complete their work.
"The key problem was to let the military strategy wag the political and diplomatic strategy. It should have been the other way round," he said.
Meyer, who was ambassador from 1997-2003, emphasised that there was a "sea change" in Washington's attitude to Iraq in the months after the 11 September attacks.
He revealed that in a telephone conversation on the day of the attacks, the then US national security adviser, Condoleezza Rice, said: "We are just looking to see whether there could possibly be a connection with Saddam Hussein."
Meyer said that up until this point, the Iraq issue was no more than "a grumbling appendix" for the US government.
