The MoD insists there are enough people in the Armed Forces to perform their operational requirements to keep Britain safe despite reports that one in five personnel is not "fit to fight".
The Sun on Sunday newspaper says more than 13,000 members of the Armed Forces have been classified as "non-deployable" meaning they cannot serve on the front line.
This has raised concerns that the UK does not have the military strength to face off foes including Russia and meet the threat of Islamist extremism.
The Ministry of Defence says individuals are medically down-graded for a wide variety of reasons most of which are minor health concerns which do not prevent personnel from fulfilling their core duties.
The newspaper reports that of the 13,223 troops marked as non-deployable, the most common reason is for muscle and joint problems.
Colonel Richard Kemp, a former Commander of British Forces in Afghanistan told The Sun: "Our forces are already cut to the bone and too small to deal with the threats we face from Islamist terrorism, Russia, Iran and unknown enemies.”
An MoD spokesman says most medical downgrades are temporary, often to allow for recovery and rehabilitation.