Three out of every five drivers are confused by the “C” symbol on road signs informing people of the Dart Charge payment system, according to the AA.

Nearly 60% of people who see the sign on the M25 take it to refer to the London congestion charge when it is supposed to indicate the free-flow payment system at the Dartford Crossing, according to a recent survey carried out by the motoring organisation.

Seven in 10 drivers in the south east are most likely to be misled by “not fully appreciating the sign’s meaning”, according to an AA Populus poll.

Despite an international campaign to raise awareness of the changes at the crossing, more than half of drivers across the UK said they were not aware of the charging scheme – including more than a quarter of motorists in the south east.

Awareness of Dart Charge was actually highest in East Anglia and Essex, where 82% of respondents said they were aware of the scheme.

The AA has argued that this may help explain why the initial failure-to-pay rate among UK drivers was 50% worse than predicted, combined with the point that road users are suddenly confronted with the sign at a busy location.

The sign, a red circle surrounding the letter C, is not officially recognised as a sign for tolled roads despite its appearance, the AA has argued.

Paul Watters, head of policy at the AA, said: "The ‘C’ sign has come to stand for ‘confusion’, primarily associated with congestion charge zone schemes in London and Durham.

"Drivers associate it with entering an area with congestion rather than a charge to use part of the trans-European road network at Dartford – one that was paid for as far back as 2003, through specific tolling.

"It is bad enough trying to use an existing sign with a specific purpose to mask what is in effect a revenue-raising scheme.

"However, presenting drivers who are unfamiliar with the Dartford stretch of the M25 with a puzzle, while they are supposed to be concentrating on a busy road, is very poor.

"The AA says that, whilst electronic charging at Dartford has alleviated some congestion, it is not the clearest way to impose road charging on drivers – particularly when the way it is done doesn’t make sense."

Dart Charge project director Nigel Gray said: "The Highway Code is clear that the red ‘C’ symbol means that you are entering a congestion charging zone.

"Dart Charge is successfully speeding up journeys at the Dartford Crossing, and the vast majority of drivers are paying for their crossings correctly.

"We continue to work hard to help drivers understand the changes we have made at the Dartford Crossing, which have already significantly improved journeys at this vital transport gateway."