Criminal gangs working across Hampstead are ordering spare keys online using numbers etched onto the access locks.

Officers have confirmed that CCTV footage of one of the thefts shows fraudsters using a key to open one of the boxes before lifting the lid with his elbows to avoid leaving fingerprints.

Residents with external mail boxes have been urged to review their security after it emerged that a number of websites will simply supply a spare key upon request when submitted with the barrel number.

“This was tested by a burglary victim last year,” said PC Ed Bromilow, ward officer for Frognal & Fitzjohns.

”Whilst not a post box, he had a very expensive front door and tested whether you could buy a duplicate key.

“All he had to do was put the make of the lock and the serial number from his key into one of many key cutting websites and he had a key that unlocked his front door in less than 48 hours.

“Granted, this was a front door rather than a post box, but the same theory applies. Quite a few post boxes have a serial number stamped onto them and from what I have seen it would suggest this or something similar is taking place.”

Police advise the use of CCTV, especially monitoring communal post boxes, in order to assist any police inquiries.

Credit checking services also provide an extra layer of security and alert customers to any new accounts which are opened.

Similarly, any past victims of fraud are advised to utilise extra CIFAS anti-fraud checks.

Anyone with any information or wishing to report a theft can call the police on 101.