Britain has "failed" in Iraq by not having enough troops and by switching resources to Afghanistan before the mission was finished in Basra, General Sir Richard Dannatt, the head of the Army, has said.

Gen Dannatt said deploying a substantial force to Helmand in 2006 left the military without the troops for any potential 'surge' in Basra as the city descended into near anarchy and under the control of rogue militias.

He admitted that "in truth, we failed to maintain the force levels required particularly towards the later end of the campaign, by which time we were already committed to a new operation in Afghanistan."

Gen Dannatt, who will retire in August, said one of the key lessons from Iraq was the need to achieve a "decisive effect" early on but this was lost without enough manpower on the ground.