The head of the Army, General Sir Richard Dannatt, said under-manning meant that almost all units were now committed to operations, training for war in Iraq or Afghanistan or on leave.

Only one battalion of 500 troops - the Spearhead Lead Element - was immediately available to deal with emergencies such as a terrorist attack, suggested the document, obtained by the Daily Telegraph.

Britain's second back-up unit, the Airborne Task Force, formed around the Parachute Regiment, was unable to deploy fully "due to shortages in manpower, equipment and stocks".

"We now have almost no capability to react to the unexpected," wrote Gen Dannatt, the Chief of General Staff, in his memo to other defence leaders.

Reinforcements for emergencies or operations in Iraq and Afghanistan were "now almost non-existent", he added.

The emergence of the memo comes shortly after Defence Secretary Des Browne's announcement on Thursday of a cut of 500 UK troops in Iraq this winter, which will leave 5,000 personnel on the ground.

But it comes at a time when additional soldiers are thought to be needed to deal with a resurgent Taliban in Afghanistan.

Mr Browne this week gave his backing to an MPs' report calling on Britain's Nato allies to supply more troops.

Shadow defence secretary Liam Fox said the lack of reserves was a "damning indictment" of the Government's stewardship of the Armed Forces.