Why the plates are being stolen
West Yorkshire Police says the plates are being taken off cars and put onto different vehicles, so that the perpetrators can fill up at petrol stations and drive off without paying.
They are also taken to replace plates on stolen cars, to prevent them being tracked by number plate recognition cameras (ANPRs).
Where it's happening
Officers said the plates are being taken in Kirklees, but motorists across West Yorkshire are advised to be vigilant.
What to do if it happens to you
If your number plate is lifted or goes missing, report it to police at once on 101 or 999 if urgent.
How to protect your vehicle from number plate theft
You can buy anti-tamper screws which put off the thieves and make it much harder to unscrew them, available at most car accessory stores, for just a few pounds.