BBC Two's Top Gear programme reveals that three family vehicles tested under controlled conditions on the programme all had inaccurate speedometers.

Yet the UK's top traffic policeman, Assistant Chief Constable Paul Manning, has proposed that even speeds of 31mph in a 30mph area should lead to prosecution. Police currently allow a small margin of error before prosecuting drivers.

Datron Technology - who calibrate police speed guns - tested a Ford Sierra, a Rover Metro and a Jeep Cherokee.

Despite being driven at a variety of speeds, none of the vehicles recorded an accurate reading.

"The Sierra's speedometer was recording 20mph while the instruments say it was really travelling at over 22mph," said series producer Julie Clive.

"If zero tolerance were to be imposed, the driver of such a vehicle could be prosecuted."

Speed cameras: More accurate speed detection equipment is being introduced

Mr Manning said new detection equipment was becoming more accurate.

But Ms Clive said: "If it makes sense that if drivers are to be fined, and possibly banned, on the basis of speeds measured with pin point accuracy by police, then they should be able to have confidence in the accuracy of their own speedometers."

The test results came as Sussex Police launched a pilot project in which they will tell companies about employees caught speeding in company cars.

Inspector Steve Long said: "Surveys show that up to a third of speeding vehicles are company owned.

"Some drivers may be under pressure to drive too fast because of the pressure of timetables and we think companies should take more responsibility."

He denied that the move infringed civil liberties.

Stuart White: "This sledgehammer approach will be ineffective"

"Speed camera tickets already go to the company so it is a logical extension of that."

The scheme has been attacked by the Association of Car Fleet Operators.

Director Stuart White said: "This sledgehammer approach will be ineffective.

"The reality is that there are good companies who are diligent and support driver training, but others do not.

"It's a shame that people who beat up old ladies do not seem to be pursued with the same vigour as motorists."