In the memo, leaked to the Daily Telegraph, Gen Sir Richard Dannatt said the Army was undermanned because of operations in Afghanistan and Iraq.
He also said vital equipment was being used "at the edge of sustainability".
A spokesman for the Ministry of Defence said the forces were working hard but the current situation was "manageable".
Gen Dannatt also said the Army has "almost no capability to react to the unexpected".
He said: "It is difficult to predict the long-term effect of this level of pressure on people. It is critical that we improve manning as quickly as we can."
There is reportedly just one battalion of 500 troops, called the Spearhead Lead Element, available to be used in an emergency, such as a major domestic terrorist attack or a rapid deployment overseas.
BBC defence correspondent Paul Wood said the Army currently comprised about 98,000 personnel, some 2,000 soldiers short of capacity.
He said the internal memo was leaked to put political pressure on the government ahead of a spending review.
"Although clearly there's politics at play here, I think this is a heartfelt plea from Gen Dannatt that if things are just about sustainable now, it cannot go on for much longer like this," our correspondent added.
The document also said that Britain's second back-up unit, the Airborne Task Force, which is formed around the Parachute Regiment, could not deploy fully.
It blamed this on "shortages in manpower, equipment and stocks".
The MoD conceded that if "operations continue at this pace, we will have to revisit our planning assumptions".
The spokesman added: "In recent months, we have drawn down our force levels in a number of operations.
"The Armed Forces' mission in Northern Ireland will end on 31 July; we withdrew the bulk of our forces from Bosnia-Herzegovina earlier this year and... we will reduce further our force levels in Iraq by 500.
"We are certainly not complacent about the longer term implications, which we are acutely aware of and are addressing."
Liam Fox, the shadow defence secretary, said the lack of reserves was "an appalling situation and damning indictment" of the way the government handled the Services.
"They are being asked to carry out tasks for which they are neither funded nor equipped. There is an urgent need to review our strategic approach because we cannot continue over-stretching our Forces."
In October Gen Dannatt said that the presence of UK armed forces in Iraq "exacerbates the security problems" and they should "get out some time soon".
He also said that initial planning for the post-war period had been poor.
Güncelleme Tarihi: 21 Temmuz 2007, 15:59