Robert Jordan was puzzled when he got charged an extra £1 for going over the Dartford Crossing. The additional amount was for a transit van when his car is a Ford Fiesta.

He queried his statement on his Dart Charge account and was refunded but there was no explanation until the charges kept coming and then the mystery was solved.

A bolt on his rear number plate meant the letter D was being read by the cameras as a B, the same registration as a transit van.

However, while that has explained the wrong bills, Mr Jordan, from Hinton Crescent, Hempstead, still cannot understand why the van owner has not been billed, and neither can Highways England, which operates the crossing.

Since the toll-free system was introduced in November 2014 it has been dogged with teething problems and even longer queues, but this particular problem is the first to come to public notice, although the authority admits it is “rare”.

Mr Jordan, who has now covered the bolt with a yellow cap, said he had kept his Dart Charge account automatically topped up and rarely checked his balance but a few weeks ago, he noticed the van charge had been applied on several occasions.

The government agency immediately apologised, paid him back the £55 overpayment and said it would not happen again but less than a week later the 36-year-old train engineer was once again charged £2.63 instead of his £1.67 payment for a car.

Mr Jordan, a dad of two, said: “I can now understand how it happened but what I want to know is why the owner of the transit van has not been getting these charges and they have been automatically accredited to my account.”

A Highways England spokesman said: “Dart Charge’s vehicle identification systems are highly accurate, even with dirty or obscured number plates, but from time to time mistakes can be made.

Mr Jordan was mistakenly charged a higher rate as the automatic number plate recognition system identified his number plate as belonging to a van instead of a car.

“Once we recognised this had happened, we immediately credited his account and updated our records to minimise the risk of the issue recurring. We are sorry to Mr Jordan for any inconvenience.

“The placing of a fixing bolt on Mr Jordan’s number plate obscures his vehicle registration number and makes it difficult to ensure 100% accuracy, even with the extra manual review of the images that we have added. Mistakes like this are rare, and whenever they come to light we take immediate action to put things right.”