Taking its toll: The Department of Transport has admitted that the barrier-free tolling system on the M50 still has a failure rate of around 10 per cent.

The system is failing to read tags or is misreading them, or is reading number plates wrongly, while human error is blamed for mismatching photos of number plates with registered tag holders. Motorists have complained about getting bills for journeys they did not make while those who tried to pay have had difficulty getting anyone to take their money.