Gen Dannatt, Chief of the General Staff, said the failure of coalition forces to take advantage of the "window of consent" in the immediate aftermath of the invasion had opened the door to the Shia militias.

He said they had not kept enough troops on the ground - particularly as the focus of operations switched to Afghanistan.

His comments, in a keynote address to the Royal United Services Institute think-tank in London, came as Whitehall is gearing up for the Government's "lessons learned" inquiry on the Iraq War.