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

But 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".

"I think ministers were aware at their level. They constantly talked to their US opposite numbers for the need for proper aftermath planning," he went on.

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.

But 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 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.