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The Met Police has been criticised for handing over the addresses of 30,000 London gun owners to a third party for a marketing campaign.

Gun owners were only made aware their details had been given out after a police campaign leaflet, posted from Leeds, was sent through their letterboxes last month, according to technology website The Register.

The website said the leaflets raised “severe” security implications for the capital’s gun owners, which it estimated to be about 5,000 rifle owners and 25,000 shotgun owners.

However the Met said the company used to distribute leaflets is a “reputable” agency specifically contracted by the police and holds appropriate security accreditation.

A spokesman added the force is investigating the complaints, but is “assured” that all personal data was deleted.

The leaflet advised gun owners to protect their firearms with ‘SmartWater forensic property marking’, a gun protection pack with a £8.95 charge for membership.

The envelopes containing the leaflets were allegedly marked with a return address for a business centre in Leeds, home of Corporate Document Services (CDS), a direct marketing company.

The Met claims the invisible ink marking system “strongly deters burglars by up to 85 per cent”.

A spokesperson for the Information Commissioner's Office (ICO) told The Register: "Businesses and organisations are required under the Data Protection Act to keep people's personal data safe and secure.”

A Met Police spokesman said: “CDS are a reputable, print company and not a commercial marketing agency.

“They are already, specifically contracted by the Met for 'the supply of external print and associated services' under a Crown Commercial Services framework.

“They hold appropriate information security accreditations and no personal information is to be retained.

“We have contacted them and the sub-contractors and we are assured that all the personal data has been deleted in line with their policies. We are investigating the matter internally and will keep the ICO informed accordingly.”