Six vehicles were stolen while they were being defrosted in South Yorkshire earlier this week.
South Yorkshire Police said six unattended vehicles were stolen on Monday and Tuesday while they were being defrosted.
A force spokesman said: "The cold frosty mornings are with us again. Please, please, please don't leave your vehicle unattended with its engine running as your car may well be missing when you come back out to it.
"Plus your insurance will be invalid if you do this, so it may well turn out to be a very expensive mistake. You have been warned."