People have been warned they could be prosecuted for seemingly harmless ‘friendly banter’, with fines of up to £2,500 or a charge of criminal damage possible.

If you spot a car that is particularly grubby, you must resist the urge to run your finger through the dirt or write ‘clean me’ or ‘wash me’ in the filth, experts have warned.

That’s because in the eyes of car obsessives, you could end up scratching the paint and causing hundreds of pounds worth of damage.

And if you do, the owner could pursue a charge of criminal damage.

Graham Conway, managing director at Select Car Leasing, said: “Some motorists will go to great lengths to protect their paintwork.

“There’s a whole ‘car detailing’ scene in the UK where enthusiasts swap advice on how to clean, polish and protect the paint on their pride of joy.

“And if you were to write ‘clean me’ on a vehicle owned by someone who looks after their car, you could find yourself in a heap of trouble.

“The issue is that swirling your finger through the mud on a car can actually scratch the paintwork.

“You can catch dirt and grit underneath your fingertip, which then acts as an abrasive as you write, potentially removing the top layer of ‘clear coat’ lacquer that sits on top of the paint.

“If you applied enough pressure while daubing a message, you might even leave the ‘ghost’ of a word on the paint, which is visible even after the owner washes the car.

“All in all, your friendly ‘banter’ could go awfully wrong if the vehicle’s owner decides to report the incident to police.”

If police get involved you could appear before magistrates to recover the cost of repairs.

If the “criminal act of vandalism” is valued at less than £5,000, then a fine of up to £2,500 can be granted.

The respray of a single car panel can cost around £500.