Admiral Lord Boyce told the Iraq Inquiry that “dysfunctionalism” existed in Washington between the Pentagon, the US State Department and the White House, which meant that officials hadn’t talked to each other about the situation.

He said US Defence Secretary Donald Rumsfeld was so obsessed with carrying out the invasion with as few troops as possible that it did not have enough “boots on the ground” and that his officials did not realise they would not necessarily be greeted as liberators by Iraqis from Saddam Hussein’s regime.

Lord Boyce, who has since retired, said: “I often found myself briefing my American counterpart on what was going on in State rather than him actually finding out directly.

“I could not get across to them the fact that the coalition would not be seen as a liberation force.

“This was absolutely not accepted.

“I think, as far as the Pentagon was concerned – both the civilians in the Pentagon and the uniforms – they just thought that Iraq would be fine on the day.”

Lord Boyce added that Mr Rumsfeld and other senior US officials had a “complete reluctance” to believe the UK would not commit troops unless the diplomatic process had been completed and the UK Parliament had approved sending troops to war.

He added: “I was always extremely concerned about the anorexic nature of the American contribution. The Americans at that particular stage were very much, ‘We’re going to do the war fighting, not the policing.”

The former military chief said the build-up to the invasion of Iraq was hampered by the inability of the armed forces to plan properly for war.

Timelines were so tight, the 7th Armoured Brigade “Desert Rats” – the lead British formation – did not achieve full operational capability until the day before British and US troops went into Iraq on March 20, 2003, he added.

Lord Boyce also demanded assurances from Prime Minister Tony Blair that the military action was legal.

At the time of the 9/11 attacks in 2001 there had been “absolutely no contingency planning” by the British for military action against Iraq, he told the hearing.

Preparations within the Ministry of Defence were “ramped up” after Mr Blair visited US President George Bush at his Texas ranch in April 2002, although they were restricted to a small group of senior officers, he added.

Lord Boyce said ministers had been concerned that details would leak out, undermining their efforts to get a new United Nations Security Council resolution requiring Saddam Hussein to give up his weapons of mass destruction.

He said he was ordered by the then UK Defence Secretary Geoff Hoon not to discuss it with the chief of defence logistics, preventing, he claimed, military chiefs from taking any practical steps to prepare for military action such as buying extra equipment.

The MoD was only finally given the authorisation to begin “overt” planning in November 2002, following the passing of Security Council resolution 1441 requiring Hussein to disarm.

He said this left the MoD struggling to mobilise its logistics organisation with the deadline looming.

But Lord Boyce said he was confident that the troops at “the front of the frontline” had the equipment they needed

when the war began.

Eventually he was given a certificate by the Attorney General, Lord Goldsmith, although he indicated that he would have preferred a second Security Council resolution authorising military action that would have “completely nailed” it.