Classic car owners could be damaging their cherished machines if left stored for long periods with unleaded fuel in the tank.

When left unused for longer times, unleaded petrol can turn into a non-flammable and corrosive liquid that attacks the car’s fuel system and engine. When an owner tries to restart their car after a period of storage, this liquid is then sucked through the engine and can cause serious problems.

Classic car restoration expert Mike Williams said: ‘Many owners of classic vehicles drive their cars only infrequently and, while they may perform some general ‘lay-up’ tasks when they put the car away for a few months, draining the fuel tank is unlikely to be one of them.

“Yet, as we and other specialists we have spoken to are seeing, this can be bad news – and it seems to be a growing problem.

“Even some manufacturers of modern road cars and agricultural equipment have issued service bulletins on the matter.”

Many older cars have fuel systems that were not designed for use with unleaded fuel. They need an additive to work properly with unleaded petrol or a new cylinder head with hardened valve seats to function. Even with this work completed, unleaded fuel can still cause difficulties.

Mr Williams explained that the ethanol which is used in modern fuels was responsible for some of the problems which are being experienced by classic car owners who leave their cars standing for long periods.

“The best cure is to use your car regularly and enjoy it,” he added.