Motorists have been urged to be extra vigilant after a spate of thefts of catalytic convertors in and around Shepton.

The pollution-scrubbing devices containing precious metals have seen them become a target for organised gangs.

Police said that while thieves once struck in large public car parks more recently they had targeted residential streets and even outside people's homes.

A spokesman said: "Thieves sometimes pose as people who are working on a vehicle. A team of three is often used – one lookout, one driver and one cutting person."

People are being urged to report suspicious activity around vehicles and take precautions including fitting a catloc device that makes it far harder for thieves to detach the unit.

In a statement,the police advised: "Precious metals such as rhodium, platinum and palladium are used in catalytic converters.

"The price of some of these metals has risen sharply in recent years and as a result has led to an increase in the theft of catalytic convertors.

"Cars that are most often targeted are hybrid vehicles, as these vehicles have two power sources, so the catalytic converter is used less frequently. The metals are less likely to corrode, meaning they are worth more and therefore more attractive to thieves.

"Hybrid cars and some Japanese makes are popular targets for offenders. Vans and SUVs are particularly at risk, as the ride height makes access to the exhaust system beneath them easier.

"On diesel cars, the diesel oxidation catalyst is often integrated with the Diesel Particulate Filter (DPF) and there is evidence that these are being stolen for their scrap value too.

"To keep your vehicle safe, ask your manufacturer or dealer if they can advise you on locks or guards to protect the catalytic converters under the vehicle.

"For safety reasons, any products will need to be approved by the vehicle manufacturer for use on your vehicle model and you should also make your insurance company aware of any vehicle modifications.

"Once an unmarked converter has been removed from a vehicle it's quite difficult to match it to that vehicle, as there aren't any distinguishing marks.

"Security marking of catalytic converters is possible with a heat-resistant marking system and is best applied by a garage when the vehicle is being serviced.

"Secured By Design member companies that provide catalytic marking systems include: "Catalytic Convertor Marking System.

"Retainerguard Catalytic Convertor Kit "Secured By Design is the only way for companies to obtain police recognition for security-related products in the UK.

"For more information visit their website."