More than £48m in unpaid tolls and fines were written off by the Dartford Crossing in 2016-17, accounts published by the government show.

The sum was chiefly made up of unpaid fines - accounting for some £42.4m - along with £6m in charges not paid.

The figures are substantially higher than the previous year, when it was forced to write off just £500,000 in charges and £10.3m in unpaid fines owed by drivers.

However, most of the debts relate to what are described as “aged debts resulting from the historic issues” - pre-dating the introduction of the Dart Charge.

The accounts state: “Write-offs were exceptionally high this year as management dealt with a legacy population of debts built up prior to the system improvements.”

It blames a combination of factors for the sum, saying: “Write-off has materialised for a variety for reasons, notably, penalty charges not issued on a timely basis, service provider error, and drivers’ vehicle keeper’s details not being available.”

The accounts say that during the 2016-17, some £22.2m “was not recognised as PCN [Penalty Charge Notices] revenue due to road users not receiving PCNs despite having contraventions recorded.”

However, steps have been taken to resolve “a significant number of the system issues and refined operational practices to address the risks of further losses.”

Despite the write-off, the charge to drivers with Dart Charge accounts saw the government collect £83.7m - about three-quarters of all revenue. A further £21.8m came from those motorists without accounts.

In a statement, Highways England said: “These latest set of accounts for the Dartford Crossing show that Dart Charge is being administered efficiently and effectively, collecting net proceeds of £61.9m during the 2016-17 financial year to be ploughed back into UK transport.

"The number of people paying on time has continued to improve from its already strong base, and the enforcement arrangements are working well.”