Thousands of British motorists are running their cars on illegal fuel - and there are fears rising oil prices could make the problem more widespread.
HM Revenue and Customs officials have carried out spot checks at petrol stations, supermarkets and car parks to catch motorists using fuels that are meant for use in agricultural machinery, and therefore aren't taxed as highly.
In some areas, one in five tanks in vehicles stopped were found to contain red diesel, which is only intended for off-road farming vehicles.
Apart from its identifying dye, red diesel is identical to normal diesel, but only costs about 35p a litre - about 65p less than ordinary fuels.
So far this year, more than 1700 private cars have been found running on the illegal fuel.
Estimates suggest that the illegal use of red diesel is costing UK taxpayers £350 million each year.
'People who use the illegal fuel are abusing the tax system. They cheat the whole country out of vital funds for public services.' said a Revenue and Customs spokeswoman.
Penalties for using red diesel illegally are imposed on a sliding scale. For instance, a person who used £27 of red diesel could be hit with a £500 penalty.
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