IT IS the ultimate back seat driver. Motorists face having their cars fitted with a “spy” device that stops speeding.

The satellite-based system will monitor the speed limit and apply the brakes or cut out the accelerator if the driver tries to exceed it. A government-funded trial has concluded that the scheme promotes safer driving.

Drivers in London could be among the first to have the “speed spy” devices fitted. They would be offered a discount on the congestion charge if they use the system.

The move follows a six-month trial in Leeds using 20 modified Skoda Fabias, which found that volunteer drivers paid more attention as well keeping to the speed limit. More than 1,000 lives a year could be saved if the system was fitted to all Britain’s cars, say academics at Leeds University, who ran the trial on behalf of the Department for Transport (DfT).

It is part of a two-year research project into “intelligent speed adaptation” (ISA), which the department is funding at a cost of £2m. Results of the initial trial will be presented to ministers this week.

A study commissioned by London’s transport planners has recommended that motorists who install it should be rewarded with a discount on the congestion charge, which tomorrow rises to £8 a day.

The trial Skodas were fitted with a black box containing a digital map identifying the speed limits of every stretch of road in Leeds. A satellite positioning system tracked the cars’ locations.

The device compared the car’s speed with the local limit — displayed on the dashboard — and sent a signal to the accelerator or brake pedal to slow if it was too fast. The system can be overridden to avoid a hazard.

“The trials have been incredibly successful,” said Oliver Carsten, project leader and professor of transport safety at Leeds University.

The DfT says it has no plans to make speed limiters mandatory but admits that it is considering creating a digital map of all Britain’s roads which would pave the way for a national ISA system.

Edmund King, of the RAC Foundation, said limiters might make motorists less alert: “If you take too much control away the driver could switch on to autopilot.”