Motorists in Wiltshire have been warned that leaving their car unoccupied while running their engine is an invitation to thieves.

As falling overnight temperatures grip cars with ice and frost, many drivers are choosing to run the engine to warm them up in the morning.

But car insurance broker the AA said thieves were increasingly targeting unwary car owners.

"Thefts of cars using the owners' ignition keys is a fast growing problem and it rises as winter sets in," said Andrew Briscoe, managing director of AA Insurance Services.

"Security systems, locks and immobilisers are becoming increasingly sophisticated and now the only way to steal many cars is to use the keys. Thieves look for cars left running on cold mornings so warming your car in this way is an open invitation for it to be stolen."

Some insurers believe that up to one in 10 stolen cars are taken because they have been left unlocked with the keys conveniently in the ignition.

And according to Home Office statistics, 60 per cent of the 375,000 cars that are stolen in the UK each year are taken from outside their owners' homes.

The AA warned that petrol stations, car washes and car parks were common targets for car thieves looking for an easy ride.

"Leaving the keys in the ignition while you fill up, pay for a parking ticket or unload the shopping is inviting trouble too," said Mr Briscoe.

"Not only do you lose your car but you'll find that few insurers will look kindly on a theft claim in such circumstances.

"Most car insurance policies will not cover theft of your car if it has been left unlocked and with the keys in the ignition.

"Leaving the engine running to warm the car up while you go back indoors to warm yourself up with a cup of tea is taking a huge risk yet it's surprising how many people do it."

Swindon police agreed, and said that cars being driven from outside peoples' homes was an issue to watch out for when the cold weather set in.

They urged drivers never to leave cars running and unattended on frosty mornings or to leave valuables such as briefcases or laptop computers in the car.

They also pointed out that it is an offence to leave a car on a road unattended with the engine running.

Nationally, up to 40 per cent of all cars stolen are never returned to their owners and those that are returned are often damaged or have valuable equipment such as audio systems or even wheels removed.

The AA has offered some simple rules to help drivers keep their car where it belongs.

If possible, keep your car in a garage it will stay frost-free and will start more readily on a cold morning. Lock the garage when you leave.

If left outside, cover your car's windscreen with newspaper, cardboard or a shield to keep the frost off.

Never start your car to warm it up and leave it unattended this is an offence if the car is on a public road.

Never start your car to warm up in the confines of a garage you could potentially gas yourself.

Keep a scraper and de-icer handy make sure you clear all of your windows and mirrors before driving off: having obscured glass is an offence.

If a de-icer is unavailable, hot (not boiling) water can be poured over your screen to defrost it but the water may turn to ice.

Always lock your car and take your keys with you when filling with petrol, collecting your car park ticket, unloading your shopping, or even popping back into your house to collect something you have forgotten.