Tony Blair made it clear to George Bush at a meeting in Texas 11 months before the Iraq invasion that he would be prepared to join the US in toppling Saddam Hussein, the inquiry into the war was told today.

The prime minister repeatedly told the US president that British policy was to back United Nations attempts to seek Iraq's disarmament, Sir David Manning, his foreign policy adviser, told the inquiry.

However, Blair was "absolutely prepared to say he was willing to contemplate regime change if [UN-backed measures] did not work", Manning said. If it proved impossible to pursue the UN route, then Blair would be "willing to use force", Manning emphasised.

Manning recalled the meeting between the two leaders at Crawford, Bush's Texas ranch, in April 2002. "I look back at Crawford as the moment that he [Blair] was saying, yes, there is a route through this that is an international, peaceful one and it is through the UN, but if it doesn't work, we will be willing to undertake regime change," Manning said.

The issue is crucial because Blair was warned at the time by Lord Goldsmith, the attorney general, and other legal advisers that going to war with regime change as the objective was unlawful and breached the UN charter.

Manning was not questioned by the Chilcot inquiry about a previously leaked document in which Manning told Blair a month earlier that he had underlined Britain's position to Condoleezza Rice, Bush's national security adviser. "I said you [Blair] would not budge in your support for regime change but you had to manage a press, a parliament, and a public opinion which is very different than anything in the States," Manning wrote.

Also in March 2002, Paul Wolfowitz, the US deputy defence secretary, was the guest at a lunch with the British ambassador, Sir Christopher Meyer. Afterwards, Meyer composed a private letter to Manning. "I … went through the need to wrongfoot Saddam on the inspectors and the UN security council resolutions and the critical importance of the Middle East peace plan. If all this could be accomplished skilfully, we were fairly confident that a number of countries could come on board," he wrote.

These documents have been handed to the inquiry, but the failure of the panel to raise them with Manning was sharply criticised by Philippe Sands, professor of international law at University College London. "I was pretty shocked by the questioning," Sands said. "I was very surprised and disappointed by the failure to press Manning on any issues".

A picture emerged during Manning's evidence of Blair resisting US pressure which began when the issue of Iraq arose in the immediate aftermath of the 9/11 attacks on America at the end of a telephone conversation between Bush and Blair. "He [Bush] said that he thought there might be evidence that there was some connection between Saddam Hussein and Osama bin Laden and al-Qaida," Manning said.

"The prime minister's response to this was that the evidence would have to be very compelling indeed to justify taking any action against Iraq."

By the time he visited Crawford in April 2002, the British were "very conscious that Iraq would be on the agenda".

Manning added: "[Bush] told us that there was no war plan for Iraq but he had set up a small cell at central command in Florida to do some planning and to think through the various options."

Blair first asked defence chiefs about the British military options for action in June 2002, the inquiry heard. Yet he delayed taking decisions until later in the year. Defence officials have said the delay was the result of Blair's concern not to give the impression that he had given up on the UN diplomatic route.

"I think there was some uneasiness in the MoD about the lateness of the decisions," Manning told the inquiry. "I personally believed [UN weapons] inspectors should have been given more time to work." They left Iraq when it was clear that the US, with British backing, was about to invade Iraq even though there was no hard evidence, despite intelligence claims, that the Iraqi leader had pursued a banned weapons programme.

One official document understood to be passed to the inquiry refers to a meeting between Blair and Bush at the White House on 31 January 2003. Blair is believed to have told the president that a fresh UN resolution Britain was pressing for was an insurance policy to provide international cover in the event of a disaster in Iraq in the aftermath of an invasion. Bush told Blair that he did not believe there would be any fighting between Iraq's different ethnic groups.

That document was not raised by the inquiry team today. However, Manning was extremely critical of the lack of planning for post-war Iraq in the US where responsibility passed late in the day from the State Department to the Pentagon.

Today's key moments

Key quote

"I was very struck by the reluctance of the US soldiers to get out of their tanks, take off their helmets and start trying to mix with the local communities," Sir David Manning said about post-invasion Baghdad.

Key jargon

De-Ba'athification. That means sacking Ba'ath party members from state jobs, something that happened when Paul Bremer took control in Baghdad after the war. Manning was very critical of the impact of this.

Damage rating

Manning was fairly scathing about the way the Americans planned, or rather did not plan, for postwar Iraq.

www.guardian.co.uk/uk/2009/nov/30/iraq-inquiry-david-manning/print