Thieves have been stealing BMW and Audi cars that were put up for sale by acting as prospective buyers.

Hertfordshire Constabulary says at least four BMW's and one Audi have been stolen in the last two months after men posing as prospective buyers viewed the cars and got hold of their key fobs.

Police say that during the viewings, the thieves managed to switch the fobs for duplicates - and that they would return later to take the cars using the stolen fobs.

The thefts occurred in Bricket Wood, St Albans, Hemel Hempstead, Borehamwood and Bushey.

Inspector Nicola Dean, from Hertfordshire Constabulary’s Crime Reduction Unit, said: “These offences were perpetrated by men who contacted the seller via online motor sales sites and arranged to view the vehicles. During the viewing the offenders have managed to switch the vehicle fobs for a duplicate.

“We advise people selling vehicles to take all the usual precautions you would when selling any item, but you should also never hand over the keys to a stranger, if you are allowing someone to test drive the vehicle make sure you accompany them and make sure you have all the vehicle fobs afterwards.”

Hertfordshire Constabulary has offered the following advice to minimise the chance of becoming a victim

• If selling a vehicle, the owner should not permit any potential purchaser to test drive the vehicle on their own, under any circumstance. Vehicle owners should also consider their own personal safety when accompanying a potential purchaser during a test drive.

• Likewise, potential purchasers must consider their own personal safety if asked to meet and view a vehicle advertised for sale, not at the sellers address i.e. a remote location or public car park. Purchasers are advised not to view any vehicle while in possession of large amounts of money.

• Make sure any test driver has a valid driving licence and suitable insurance cover. You could be liable for any accidents they may have.

• To avoid buyers being left alone (and potentially driving away) with your vehicle, keep hold of their keys at all times and avoid leaving them in the ignition.

• Never hand over the vehicle keys or documentation until your bank has confirmed the full value of the vehicle has cleared into your bank account.

• Don’t be pressured into releasing your vehicle – a genuine buyer will not mind waiting until the draft has cleared.

• Be careful about how you take payment. Ask for cash to be handed to you in a bank, where the notes can be checked for forgeries and paid in immediately. Online bank transfer is one of the safest ways to pay as it avoids handling large amounts of cash.

• Obscure the number plates in the photo(s) you use to advertise your car. If a potential buyer asks why, explain how plates can be cloned for use on other vehicles for more serious crimes.