George Bush tried to make a connection between Iraq and al-Qaida in a conversation with Tony Blair three days after the 9/11 attacks, according to Blair's foreign policy adviser of the time.

Sir David Manning told the official inquiry into the war that Bush, speaking to Blair by phone on 14 September 2001, "said that he thought there might be evidence that there was some connection between Saddam Hussein and Osama bin Laden and al-Qaida."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," Manning said.

Blair followed up the conversation with a letter stressing the need to focus on the situation in Afghanistan, where the attacks originated.

But by the time Blair went to visit Bush at his ranch in Crawford, Texas, in April 2002 the British were "very conscious that Iraq would be on the agenda", Manning said.

Manning, who was firmly ensconced in the Blair camp in the run up to the war, said both he and the prime minister told Bush it would be "impossible" for the UK to take part in action against Iraq unless it was through the UN. The UK's subsequent participation in the invasion alongside the US without UN backing was because Blair felt he had to be "as good as his word" to the US, Manning said.

Manning, who became ambassador to Washington in 2003, told the inquiry he personally "regretted" that diplomacy and weapons inspections were not given more time and that he thought it was "essential" to have a second UN resolution. But the US was convinced these measures were not working and that a second resolution was impossible.

Manning said Blair was so intent on diplomacy that he may even have affected military planning and there was probably "some uneasiness in the MoD about the lateness of decisions".

"There was a sense in the MoD, probably, that we had to try and ensure that the policy that we were following diplomatically did not mean that we were excluding military options," said Manning, stressing that he was not a military expert. "My impression was that he [Blair] was reluctant to take these decisions until he had to, that some might have said he went beyond the ideal of when he had to, he left it quite late. But I think he always felt that he wanted to give the sense that the diplomatic approach in the United Nations was paramount."

The original plan, until the end of 2002, was for British troops to go into northern Iraq. The British had to adapt their plan when in early 2003 Condoleezza Rice, Bush's secretary of state, called Manning to say Turkey was refusing to co-operate.

Asked about the "conditionality" Blair was supposed to have demanded from the US in return for Britain's co-operation, Manning said Blair pressed for a Middle East conference that did not happen.

He said postwar planning was a matter of concern and the British had insisted on a role for the UN. The "neocon wishful-thinking thesis" was that postwar Iraq would be like postwar Germany or Japan, said Manning. The British had thought the US state department would be in charge for postwar planning but it was the Pentagon that took responsibility.

"The Americans seemed to lose focus" after the initial overthrow of Saddam's regime, said Manning. He described as "limited" Britain's influence on the Coalition Provisional Authority, run by Paul Bremer, which disbanded the Iraqi army and purged members of Saddam's Ba'athist regime.