Police have warned drivers they could be fined up to £1,000 if they allow their registration plates to become covered in dirt.
The warning, from North Yorkshire Police, comes after officers stopped a car with a "totally invisible" number plate.
Insp Martin Metcalfe of Richmondshire Neighbourhood Policing Team said: "It’s not [the] crime of the century, I agree. And before anyone says it, yes, we do have better things to do!
"I’d rather people kept their plates readable than we had to spend valuable time stopping and dealing with them. But we can’t turn a blind eye if your number plate is this dirty."
He said officers had stopped a car because "not a single digit was visible" on the rear number plate.
A clean number plate means police can check a driver's insurance, tax and MOT are up to date and whether the car has been stolen or used in criminal offences.
In one recent example a vehicle stole from the Melsonby area of Richmondshire was recovered following a number plate check in the Cleveland area.
Insp Metcalfe said: "All we’re asking people to do is keep their number plates clean.
"It only takes a few seconds before you set off."
Drivers who fail to comply can be given a verbal warning and told to wipe their plate clean. But in the most serious cases they can face a maximum penalty of £1,000.