Hundreds of thousands of drivers have yet to pay the Dart Charge since it was introduced at the Dartford Crossing, it has emerged.

The Highways Agency has revealed around two million crossings have taken place during the chargeable period since the free flow system began on November 30.

But only 85% of crossings have been paid, leaving around 300,000 journeys unpaid.

Penalty notices for motorists who have used the crossing but have not paid for it have started to be processed.

Those who receive a fine will also have a warning letter giving an extra 14 days to pay the original crossing charge without a penalty.

Highways bosses have celebrated the impact of Dart Charge, which now has more than a million vehicles registered for accounts.

The say average journey times have been cut by more than nine minutes southbound, and more than four minutes northbound when comparing the first two weeks of December and November.

Project director Nigel Gray said: "Dart Charge is off to a fantastic start, and I’m really encouraged that more than a million vehicles have been registered for accounts.

"Traffic flow is going well too – there has been a real improvement in journeys – especially southbound where drivers no longer have to stop at a barrier at all.

"The introduction of Dart Charge has been a big change so it is great that the vast majority of drivers have paid the charge. We want to give all drivers the opportunity to pay and comply with the scheme.

"That’s why the first penalty charge notice issued for any vehicle for non-payment of the charge will offer the driver an extra 14 days to pay - and pay for any crossings they have made since.

“This measured approach strikes the right balance between being clear to drivers they need to pay Dart Charge and giving them every opportunity to do so."

Under Dart Charge, drivers no longer pay the crossing charge in cash at a barrier. Instead, they pay online, by phone or at payzone retail outlets, either in advance or by midnight the day after they have used the Crossing.

Work to remove the booths and improve the road layout will continue until spring 2015. For more information visit www.gov.uk/highways/dartford