A FREE-FLOW system at the Dartford Crossing is estimated to cost around £42.3 million.

The prospective price-tag was revealed in a notice in the Official Journal of the European Union ahead of the scheme being put out to tender to find a service provider.

The Highways Authority is looking for a service provider to design, build and operate the introduction of free-flow charging at Dartford, which would ease congestion of the current toll booths by replacing them with electronic payment.

A spokesman for the Highways Agency said: “Reducing congestion at the Dartford Crossing is a top priority for the Government, and the introduction of free-flow charging at the crossing has the potential to help improve traffic flow.”

A supplier briefing event is planned for companies interested in bidding for the service in central London on February 1 and the tender could go to a single contractor, a consortium or a prime contractor using subcontractors.

The Highways Agency spokesman added: “In taking forward this significant and major project, the Highways Agency is engaging with technical experts from across the industry to develop the best ideas and to gauge interest from potential suppliers.

"This will help us to ensure that we deliver these vital improvements successfully, building on best practice from similar projects around the world.”

Whichever company secures the tender will be responsible for putting in place a vehicle detection system to charge vehicles and enforce the charge, a suitable IT system and back office processing to deal with charging, discounts, registration and enquiries and complaints.

Payment methods could include online, SMS as well as contact centre and retail options.