Filling your car's tank with the wrong type of fuel may be an easy mistake to make - but this temporary lapse in concentration will not only ruin your engine, it could also hit your pocket hard. New findings suggest there are 150,000 annual cases of mis-fuelling in the UK (*), a figure that is rising due to the growth in diesel car sales (**). Owners of diesel vehicles are more likely to mis-fuel than petrol drivers: 95 per cent of mis-fuelling cases involve motorists putting petrol in a diesel car because of the wider diesel filler neck and narrower unleaded nozzle. (*) Diesel nozzles, on the other hand, are generally too wide to fit unleaded tanks.

Counting the cost

Filling a diesel car with unleaded can cause the vehicle either to break down or suffer major engine damage. The cost of repairs can range from £300 to drain the tank, to more than £5,000 if the car's engine fails after being driven with the wrong fuel (*). But while many drivers assume their insurance company will pick up the tab for the damage, this may not be the case. Research shows that nearly a third (*) of insurers would not pay out for mis-fuelling claims.

Don't attempt to drive

If you've put the wrong fuel in your tank, do not turn on the ignition or start the engine, as this will circulate the contaminated fuel and increase the risk of danger - and may mean an insurer will refuse to pay out. Motorists who knowingly drive their cars with the wrong fuel in may be deemed to have failed in their duty of care - which could mean a claim of up to £5,000 being turned down.

Help is at hand

If you do fill your car with the wrong type of fuel, notify the petrol station and breakdown company immediately. The AA, for example, has a dedicated roadside service called Fuel Assist which drains, flushes and replenishes car fuel systems on the spot.

Check your cover

Insurers which do cover mis-fuelling claims - including the AA - usually class it as "accidental damage" under a comprehensive insurance policy. "But if it was clear you had driven any distance knowing you had mis-fuelled, you might invalidate the claim," says Ian Crowder from the AA. More Than would also cover the cost of repairing the engine damage caused. "But if you broke down as a result and needed assistance, you would need to have breakdown cover in place to rescue you, as this wouldn't be covered in the policy," says More Than's Lana Clements.

Watch out for mis-fuelling exclusions

As a motorist, you need to be aware that if the policy is third party, fire and theft - or contains a mis-fuelling exclusion - you are unlikely to be able to claim at all. Check your policy documents carefully so you know what you are covered for. If you're not sure, ring your insurer. "Nobody is immune to mis-fuelling and it is easily done," says Graeme Trudgill from the British Insurance Brokers' Association (BIBA). "But it is important to arrange a fully comprehensive policy that covers these unexpected risks."

Making a complaint

If you feel that the mis-fuelling exclusion was not brought to your attention or included in the documentation when you bought your motor insurance, you have the right to complain to the independent Financial Ombudsman Service.

New mis-fuelling prevention device

If you want to buy a device to prevent mis-fuelling, then the RAC has just launched FuelSure, a product designed to make it impossible to put petrol into diesel vehicles. The device acts as a cap over the diesel filler neck which can only be removed by inserting a diesel refuelling nozzle and twisting the cap off. It is currently on sale for £29.95. Notes: (*) The British Insurance Brokers' Association (BIBA) (**) AA