The introduction of the Dart charge at the Dartford Crossing saw revenue increase by £61.8 million in 2016 - the first full year of the new payment method.

Accounts for the financial year 2015-16 show revenue raised from charges was £161.1m compared to £99.8m in 2014-15, when the tolls were replaced.

But despite the hike, auditors say there were losses of £33.6m because Highways England had to abandon efforts to recover unpaid charges because of "significant issues" with the system used.

As a result, in many cases too much time had elapsed to recover money owed by motorists who had failed to pay.

In a critical report, auditors say: "Data held to support statutory enforcement activity was not complete and accurate.

"As a result, Highways England decided to freeze or abandon the issue of Penalty Charge Notices to certain customers during this period.”

The report says "the figure is unusually high for this year as a result of system issues encountered during the first half of 2015-16."

It adds while many of the problems were "substantially resolved" the temporary measures to limit enforcement activity had led to a loss for taxpayers.

But it says while the figure was "unusually high" the estimated losses were probably about £8.8m taking into account the fact many drivers would have been first time offenders who would have been given the chance to pay within 14 days.

On the increase in revenue in 2016, auditors say there has since been tougher enforcement action against those who fail to pay the charge along with more traffic.

The crossing suffers regular delays despite the new charging system, leading to frequent congestion and delayed journeys during the rush hour.

The net proceeds for the company that operates the scheme, Emovis Operations Leeds Ltd, were £57.2m compared to £12.2m in 2014-15.

The RAC said the government should be directed towards funding the third Thames crossing.

A spokesman said: "These are big numbers and drivers will want to know where all the cash ends up.

"There was a wide expectation that tolls would disappear after the QEII Bridge was paid for back in 2002.

"That didn’t happen and drivers continue to fork out for what is now called a 'congestion charge' with any money raised having to be spent on transport schemes.

"Road users travelling between Kent and Essex will expect a large amount of the profits to be used to fund the new Lower Thames Crossing."

The Highways Agency issued a statement saying the system was being administered effectively.

"The 2015/16 financial year was the first full financial year that the Dart Charge scheme has been active, and the accounts show that it has been administered effectively.

"Sir Amyas’ [the auditor] notes are detailed in the report. He also notes that the underlying issues have been substantially resolved and that the accounts are being published without qualification."

On the question of why Kent did not benefit directly from revenue raised from the charge, the agency said: "By law, all income collected from the road user charging scheme at the Dartford Crossing has to be spent on transport.

"It is taken into account as part of the Department for Transport’s overall financial allocation and must be spent on transport-related schemes and projects.

"All cash receipts are passed over gross to the Department for Transport."