Defence chiefs have urgently recalled decommissioned rifles from insecure cadet units amid fears they could be converted and turned into weapons used by criminals and terrorists.

In a letter seen by The Telegraph, the Ministry of Defence (MoD) has warned that almost 10,000 rifles used by cadet units “were not deactivated according to Home Office guidelines [and] have the potential to be converted to live firers”.

The letter, sent by Headquarters Regional Command, refers to a break-in and theft of a number of so-called ‘drill purpose’ weapons. It calls for units as a matter of urgency to inform the command centre, based in Aldershot, how long it would take to move the weapons to “alarmed armouries”.

The recall of the weapons is thought to have been triggered by the theft of firearms from a Fife army cadet unit, based in a wooden hut, in May last year. Three training rifles were taken in the theft.

Police tested the weapons, which were recovered in the summer, and discovered they could easily be converted to live firers, at which point the MoD was informed.

The Telegraph understands it was the Police Scotland tests that led to the urgent recall as the weapons could be easily turned back into live assault rifles “with the right tools and knowledge”.