Tony Blair himself raised concerns directly with President George Bush amid alarm in Whitehall at the state of the Pentagon’s preparations.

However, senior figures in Washington had a “real blind spot” and assumed there would be “dancing in streets” when the invasion took place, senior diplomat Edward Chaplin told the inquiry. “We tried to point out that was extremely optimistic,” he said.

Mr Chaplin, who was head of the Middle East section of the Foreign Office at the time of the March 2003 invasion, said there was “a pretty dire state of lack of planning”.

There was “a touching belief [in Washington] that we shouldn’t worry so much about the aftermath because it was all going to be sweetness and light,” he said.

“I think ministers were aware at their level. They constantly talked to their US opposite numbers for the need for proper aftermath planning.”

Mr Chaplin, who became the British Ambassador to Baghdad in 2004, said the US State Department had initially started the planning for after the invasion. Its work was discarded when the task was taken over by the Pentagon – which was hostile to United Nations involvement – as the invasion drew closer.

Asked whether the Pentagon took steps to involve Britain in the planning, Mr Chaplin said: “They didn’t take many steps to involve their own colleagues in the administration in planning”.

He added that Pentagon officials were happy to listen to their British counterparts but that the UK’s ideas never had “traction”.

“These points were made at all levels, including and up to the Prime Minister talking to President Bush,” he said.

Whitehall started planning for an Iraq without Saddam Hussein in the autumn of 2002, another senior Foreign Office official told the inquiry.

Sir Peter Ricketts, director-general political between 2001 and 2003, said: “It wasn’t clear then exactly what scenario there would be. But we assumed from that point onwards that we would be dealing with an Iraq without Saddam Hussein in the aftermath of a military intervention.”

Sir Peter, now the top civil servant at the Foreign Office, said ministers had urged the US to allow more time for planning.

In January 2003, the British perceived that the US “tempo” on the invasion was accelerating, he said.

He added: “We made a major effort from the PM downwards to make the case again ... to make the case for more time. I don’t think we said six months or three months were essential, but we were certainly feeling that more time was needed.”

Sir Peter said the Foreign Office stressed to Downing Street the need for post-invasion planning to be taken “very seriously indeed”.

“We were very doubtful about the neo-con assumption that international forces would be welcomed as liberators,” he said.

Asked whether No 10 listened to the advice, he added: “Absolutely, and Mr Blair and the Foreign Secretary [Jack Straw] in their many conversations always made a point, I think, of stressing to the US that they must take planning for post-conflict Iraq just as seriously as planning for any military operation.”

Sir Peter said the body set up by the Pentagon to oversee reconstruction – the Office of Reconstruction and Humanitarian Assistance – had been poorly prepared.

“ORHA started with very little time and very little expertise, very few people on what turned out to be an enormous undertaking,” he told the inquiry.