Some 5,000 soldiers and officers are unfit to fight on the frontline, figures obtained by the Conservatives revealed.
They are unable to deploy for combat duties because of physical or mental injury or illness, a lack of fitness, or non-medical reasons, according to the Ministry of Defence data.
A number of cases included troops aged under 18, pregnant soldiers, those excused from combat duties on compassionate grounds and personnel facing disciplinary action.
The MoD said some of those classed as not fully deployable could still work on the frontline, providing suitable medical support was available in theatre.
Conservative MP Bernard Jenkin, who obtained the figures in a Parliamentary written answer, told the BBC: "To have 20 per cent of the infantry unfit for the duties they are primarily employed and trained for is quite a staggering figure.
"This reflects the long-term effect of sustained operations, and it's worth remembering the government may pay for extra ammunition and other costs of operations, but they don't fund the recruitment and training of personnel to replace those who are left unfit for combat."
Shadow Defence Secretary Dr Liam Fox said: "The infantry is the tip of the spear of the conventional Army. This is the same sad story resulting from a decade of Labour's neglect of our Armed Forces and there is no sign of improvement. This situation is not sustainable."
An Army spokeswoman said: "All units deploy at the required strength for the tasks they are asked to fulfil during their operational tour.
"The majority of those classed as medically non-deployable are fit enough to work in some capacity and therefore continue to make a contribution to the effectiveness of the Armed Forces."
The Ministry of Defence is looking at whether some servicemen and women injured so seriously they cannot return to the front line could be discharged on medical grounds.
However, officials said they would attempt to place wounded personnel in alternative Army roles where possible.
The Army spokeswoman said: "The Ministry of Defence is committed to providing excellent care for our injured personnel, and ensuring that individuals get the support that is most suitable for them.
"As well as outstanding clinical care the Ministry Of Defence provides a comprehensive range of welfare support to the injured and, recognising the vital role they play in recovery, their families.
"We have made many positive changes over recent years but we are continuing to look for ways to improve the support we offer."
Mr Jenkin told the BBC Radio 4 Today programme: "There are probably over 1,000 who are actually injured, but many others medically unfit, probably because of what they have done to themselves on operations - they have got a bad knee or they have lost their hearing and they can't be deployed on the frontline.
"This is basically running down the Armed Forces. We are more deployed than we can afford to be and, crucially, the Treasury doesn't pay for the replacement and training of people who are killed or injured on active service. It is not funded by the Treasury as part of the cost of operations.
"This goes right to the heart of what the defence review will be about. It is about matching commitments with resources. Our commitments are way ahead of our resources on a sustainable basis at the moment."
Colonel Richard Kemp, a former commander of UK troops in Afghanistan, told BBC Radio 4's Today programme: "I am not surprised by this. It is a situation that has been steadily deteriorating, largely as a consequence of the increasing intensity of operations in Afghanistan.
"In my view, the picture is even worse than it has been painted by Bernard Jenkin, inasmuch as a disproportionate number of these soldiers come from the fighting element of an infantry battalion
"An infantry battalion is about 650 men - 450 of them are actually fighters, as opposed to logisticians and administrators, so of those fighting soldiers who are closing with the enemy and killing Taliban in Afghanistan, the figure is probably closer to 30% of them who are unavailable.
"What we need to do is make more money available to increase the manpower. We need more infantry battalions. Infantry battalions have been run down over a number of years.
"They have been raided for manpower. It is an easy target to make savings and that is what has happened, so we have got very few infantry.
"The situation we have got now is being made worse because the fighting is continuing, we have got more casualties, and at the same time the Army is coming up towards full manning.
"We don't have the funds to go over manning levels and recruiting is being slowed down because so many people are in the system who can't be deployed on operations."Reuse content