By Chris Irvine
In the past five years, Essex police have had to pay out 222 times for repairs caused by its drivers putting petrol in diesel engines or vice versa.
To combat the problem, the force has fitted large yellow flaps over the filling point, reminding the drivers which fuel to use.
The vehicles have also been fitted with voice reminders telling drivers "this is a diesel vehicle" when the fuel cap is opened, while they have begun fitting devices which fit into the filler neck making it almost impossible to get a petrol nozzle into a diesel tank.
A police spokeswoman said: "Essex Police, like many other organisations, have previously suffered from wrong fuels being put into tanks.
"Part of the problem is that modern diesels are so good and quiet that drivers can be easily fooled into thinking that they run on petrol.
"Police officers and staff often drive a variety of vehicles during the course of the week and they may be either petrol or diesel fuelled."
John Franklin, spokesman for the RAC said using the wrong fuel was a common problem.
He added however: "You would think the police should be properly trained to know what fuel a car needs.
"If you are going to be using numerous types of cars, then it needs to be clear, but I don't know whether talking petrol caps are really necessary."