Police officers mistakenly filled patrol cars with the wrong type of fuel nearly 300 times last year, costing more than £50,000 in repairs.
The figures, released after a Freedom of Information investigation by the Press Association, were described by the TaxPayers' Alliance as "staggering".
Of the UK's 45 police forces, 40 responded and 33 admitted paying for repairs to a police vehicle after a misfuelling incident last year, at an average cost of £178 a time.
Some 299 incidents of misfuelling were recorded, costing a total of £53,337.
John O'Connell, chief executive of pressure group the TaxPayers' Alliance, said: "It's staggering that such a simple mistake is being made almost daily.
"This careless attitude shows a lack of respect for those same taxpayers who both pay their wages and are forced to pay for the repairs.
"Millions of people manage this task with their own cars by taking a modicum of care, police officers should extend the same courtesy to their vehicles."
A spokesman for the National Police Chiefs' Council said: "The misfuelling of police vehicles is relatively rare, especially when you consider that forces have thousands of vehicles which are in constant use and require frequent refuelling.
"Police drivers will often move from one vehicle to another depending on operational requirements, which can increase the possibility of mistakes being made.
"Some forces are already using electronic systems which have eradicated misfuelling completely, and nationally we are implementing similar technological improvements where it is deemed to represent best value for money."
City of London, Durham, Greater Manchester, Lancashire, Lincolnshire, Nottinghamshire and Dyfed-Powys did not misfuel any cars.