Conservative defense critic Liam Fox said the military was being overstretched in missions in Afghanistan and Iraq and was endangering the safety of personnel, The Daily Telegraph reported Monday.
A parliamentary defense committee report said one in 14 infantrymen is "unfit to deploy," either because of injury or illness, which translates to the army being able to field 90,000 men out of a required total of 101,800.
The military has said it is 3,800 people short because of soldiers quitting the service.
Britain is scheduled to send new troops to Afghanistan this spring but the newspaper said at least six battalions will be sent to do the job of four because of sick or injured soldiers.
The figures are based on data from 10 battalions that were in Iraq or Afghanistan last year or now in service, the newspaper said.