Two cars were stolen as the owners left the engines running unattended to defrost them.
It happened in Stony Stratford yesterday morning.
And with sub-zero temperatures expected to continue, police are warning that opportunist thieves could be on the look out to take advantage of the cold weather.
Officers say it only needs a moment for a thief to have enough time to jump behind the wheel and drive off with your car.
A police spokesman said: “The simple message is not to leave a running car unattended for any length of time whether it is left on your own driveway to de-ice on a cold winter morning or for example while you pop to post a letter.
“Give yourself extra time to defrost all your cars windows and don’t be tempted to leave the car even for a few moments.
“Modern cars are quite difficult to steal without the keys so criminals are looking for opportunities to steal the car with the keys.
“A car unattended with its engine running is a golden opportunity for a thief.
“Potentially you could lose thousands of pounds because leaving your car with the keys available in this way may leave you uninsured for the theft.”
Further information regarding crime prevention can be found at www.thamesvalley.police.uk or www.safermk.co.uk