The warning by General Sir Richard Dannatt, chief of the general staff, to fellow defence chiefs comes at a time when the army is asking for a big increase in reservists to be deployed in Afghanistan, reflecting a crisis in Britain's armed forces.

In a secret memorandum he says: "We now have almost no capability to react to the unexpected." Reinforcements for emergencies or for operations in Iraq or Afghanistan were "now almost non-existent".

He adds: "The enduring nature and scale of current operations continues to stretch people". Gen Dannatt warns the army had to "augment" 2,500 troops from other units for operations in Iraq and Afghanistan to bring up the total force to the 13,000 needed there. This remained "far higher than we ever assumed", he says.

He continues: "When this is combined with the effects of under-manning (principally in the infantry and Royal Artillery) and the pace of training support needed to prepare units for operations, the tempo of life in the Field Army is intense." While the current situation was "manageable", Gen Dannatt said he was "concerned about the longer term implications of the impact of this level of operations on our people, equipment and future operational capability".

The general's warnings, in a document leaked to the Daily Telegraph, come at a time when the Ministry of Defence and the Treasury are finalising spending plans for the next three years in the comprehensive spending review expected to be revealed in October. The three service chiefs are arguing among themselves about how the money should be spent. The navy is expected next week to be told it will get two new aircraft carriers at a cost of between £2bn and £3bn and fewer destroyers and frigates than it wants.

Gordon Brown, the prime minister who as chancellor fought regularly with the MoD, has told defence chiefs to sort out a deal among themselves, according to defence sources.

As the army has been forced to call up 600 reservists for Afghanistan there is only one Spearhead battalion of 500 troops, available for an emergency.

Gen Dannatt's memo says Britain's other rapid deployment unit, the Airborne Task Force, made up mainly of Paras, was unable to fully deploy "due to shortages in manpower, equipment and stocks".

It is not the first time Gen Dannatt has expressed concern about pressure on the army. Soon after he took up his post last August he told the Guardian that the army was "running hot" and called for a national debate on defence.