The first sharp bite of winter has seen the first cars taken away by thieves who are on the look-out for vehicles being warmed through with the engine running.

AA Insurance has warned that car owners who start their car on a frosty morning and then go back indoors, even for a moment, leave themselves open to the vehicle being stolen.

"Unfortunately, they will get a cold reception from their insurance company as ‘loss by leaving keys in an unattended vehicle’ is specifically excluded from motor insurance policies," explains Simon Douglas, director of AA Insurance, Britain's leading car and home insurance broker.

Douglas points out those modern cars' are almost impossible to steal without the keys. "To car thieves, frosty mornings are a gift," he says. "Organised criminals cruise suburbs looking for the tell-tale plume of steam rising from the exhaust and if the car is unattended, it takes only a few seconds for it to vanish.

"Such cars – especially upmarket models – are often stolen to order and are unlikely ever to be recovered unless they are equipped with a tracking device."

Thieves look for any opportunity to steal cars where the keys may have been left in the ignition: for example at filling stations or at pay-and-display car parks where the owner may have left the car to obtain a ticket.

"Keys are the weakest link in the car security chain," Douglas observes. "Never, ever leave them unattended – you don't know who is watching and waiting for just that opportunity. In such cases, an insurance claim will almost certainly not be met."

Another common technique is ‘hooking’ car keys from a hall table or key rack near the front door, using a pole or fishing rod through the letterbox. "It's unwise to leave any keys near the door," Douglas warns. "If your car is being targeted, thieves will go to considerable lengths to obtain the keys and whisk you car away."