Gangs have threatened IT staff and have walked away with millions of pounds worth of equipment in raids on large city firms, including Deutsche Bank, newspaper offices and local authorities.
In the most recent attack, three hooded men broke into the IT department at Barking and Dagenham Council and threatened and tied up IT staff, before fleeing empty handed. The Sun equipment they were looking for had been taken in an earlier raid.
Insurers are now advising IT departments to install protective security measures costing upwards of £10,000 to protect their equipment and to deter raiders.
In a circular to its customers, Zurich Municipal recommended that IT departments install security equipment including smoke cloaks, steel casing, intruder alarms and CCTV cameras.
Sun equipment should be stored in a secure room made of substantial brickwork, preferably above ground level and in the centre of the building, the insurer said.
Zurich also urged IT departments to dispose of Sun packing as soon as possible and not to leave it on view near dustbins, where it can alert thieves.