Toll booths at a river crossing on Britain's busiest motorway are being scrapped today and replaced with an electronic payment system.

The changes at the Dartford Crossing on the M25 east of London are designed to improve traffic flow.

But the removal of the 27 toll booths at the southern end of the crossing will result in roadworks until March 2015.

And car drivers using the crossing will from today face a higher daily charge - with the fee, also payable by phone or post as well as online, going up from £2 to £2.50. Charges are also rising for goods vehicles.

A Department for Transport spokesman said: "The increased charge is essential for a much-needed upgrade to the crossing which will improve journey times, and road users who register online save up to one third each time they use the crossing.

"The changes we are making will benefit millions of road users who use the crossing every year."

Under the Dartford-Thurrock Crossing Act of 1988, toll paying should have stopped when the crossing had been paid for - a date deemed as March 31 2002.

But charges have been kept on under a separate year-2000 Transport Act with the funds raised going to the Treasury where they are ring-fenced for transport purposes.

AA president Edmund King said: "The removal of the toll barriers is welcome but frankly they should be removing the toll charge.

"There should be no demand management tolls on the M25 as the bridge was paid for a decade ago. This road is London's bypass and a Trans European route. The removal of barriers will ease congestion but only after major roadworks and a 25% hike in tolls."

RAC Foundation director Professor Stephen Glaister said: "The crossing is the motorway bottleneck of Europe and routinely sees traffic volumes way above the maximum design capacity.

"We welcome the changes but they are not coming cheap. Charges are being hiked and car drivers will pay a quarter more for the privilege of faster journeys."

The Highways Agency is hoping for a minimum of congestion at the start of the new system.

An agency spokesman said: "We're starting at 6am on a Sunday when traffic will be comparatively light and there will be plenty of signs up to tell people about the new arrangement.

"Monday morning will be more of a test but we are trying to do everything we can to let people know about the changes."

The new payment method is being called Dart Charge.

Highways Agency Dart Charge project director Nigel Gray said today: "Dart Charge marks the start of a new chapter for the Dartford Crossing.

"It is the first step in speeding up journeys for the tens of thousands of drivers who use the crossing each day.

"Over the coming months we will be transforming the road layout and when this work is complete in spring next year drivers will feel the full benefits of these improvements."

He went on: "We all want to see less congestion and quicker journeys at the crossing and we are on course to deliver that, which is great news for drivers and great news for the economy.

"More than 200,000 people have registered for pre-pay accounts and from today will save up to a third on every crossing.

"It is the easiest and cheapest way to pay Dart Charge and I encourage anyone who uses the crossing to register for savings at www.gov.uk/dart-charge."