CARS will spy on each other to catch drivers trying to evade road tolls under a scheme being proposed to enforce nationwide congestion charging. More than a million vehicles would be fitted with cameras to photograph the numberplates of those who failed to pay.

The Government is planning a “pay-as-you-drive” system under which fuel duty and road tax could be replaced by tolls for each mile travelled.

The rates would vary according to the level of congestion, with drivers paying £1.34 a mile on the busiest routes but only 2p a mile on quiet roads. A satellite tracking device in each vehicle would monitor its movements to ensure that the appropriate rate was being deducted from the driver’s pre-paid account; but ministers have acknowledged that concerns over privacy remain a significant stumbling block.

A Department for Transport (DfT) feasibility study last year concluded: “The management and handling of such data inevitably gives rise to concerns about privacy.” It said that a “sizeable minority” of the population had “strongly held beliefs on the subject”.

Motoring groups liken the system to having Big Brother sat in the passenger seat, watching where each driver goes. But a team of Cambridge University scientists has designed a way to enforce road tolls that does not require records to be kept of every driver’s movements.

At least 10 per cent of cars, including more than a million police and local authority vehicles, would be equipped with cameras capable of identifying numberplates.

The “spy cars” would use radio signals to check whether the car driving in front had paid the toll for the road on which they were travelling.

If the spy car failed to detect a digital receipt, it would photograph the numberplate and store the image, together with the location and time, on an onboard computer. The data would then be transmitted to one of the roadside receivers that would be positioned every few miles across the network.

The enforcement authority would be likely to receive several reports about the same driver, allowing it to build up a convincing body of evidence of evasion before issuing a penalty. A single detection would probably be ignored as it could be the result of a technical error. The authority could decide to take action only when it received five or more reports from different spy cars about the same vehicle.

Robert Harle, research associate at Cambridge University’s Computer Laboratory, said: “Privacy would be maintained for all law-abiding motorists because the system would identify only those drivers who had failed to pay.”

Speaking at a Cambridge-MIT Institute conference last week, Mr Harle said: “People could pay anonymously for their journeys and the Government would not know where they had travelled. Rather than Big Brother, we would have peer-to-peer security, with cars checking on each other.”

He said that drivers who had not paid would escape detection on some journeys because they would not encounter any spy cars. “But that would not matter because it would only tend to happen when a driver was on empty roads,” he said. “As the whole purpose of the charging system is to tackle congestion, the enforcement needs to be fully effective only on busy roads.” While the detection system would operate automatically without the driver’s involvement, Mr Harle acknowledged that incentives would be needed to persuade private motorists to have it fitted to their cars. He said that the computer at the heart of the system could be adapted to include other functions, say, playing DVDs or providing satellite navigation. “There will be plenty of volunteers when drivers realise all the other benefits,” he said.

Professor Frank Kelly, the DfT’s chief scientific adviser, said: “This is an interesting idea which could solve the privacy question. People might be happy for a supermarket to know their buying habits from their loyalty cards but they may object to the Government knowing their movements.”