Video content Video caption: Secret filming shows drivers filling up with red diesel Secret filming shows drivers filling up with red diesel

Finally, motorists in Birmingham and Sheffield have been confronted by a BBC reporter who secretly filmed them buying red diesel to use in their own cars.

It's illegal for regular drivers to use the fuel, which is about 50p cheaper than a litre of normal diesel.

HM Revenue and Customs estimates that the crime costs the UK £100m a year in lost tax revenue.

Red diesel is intended for use by farmers and construction workers in tractors and diggers.