A source close to the...

A source close to the police claimed: "Officers say they sometimes find the technology frustrating because of the number of false or useless positive "hits" it flags up. On occasions, this means the system simply gets turned off."

Certain patrol cars have hi-tech cameras, which can read the number plates of vehicles in front or behind. An onboard computer analyses the image, and cross-references the registration to DVLA and insurance databases to see if the motor is untaxed or uninsured. The reg is also run through the Police National Computer to check if the car is stolen or the keeper wanted for any motoring offences or more serious crimes. If there is a problem, the officers are warned via a dash-mounted LCD screen, similar to that of a satellite-navigation system.

Our insider, who wanted to remain anonymous, explained: "Some data on these databases is incorrect or out of date. This means police can end up pulling over innocent drivers, which is a waste of time for both parties." He explained that one possible cause for false alarms was stolen cars not being removed from the force"s "missing" list. Alternatively, a vehicle may be listed as uninsured, but the motorist is covered through another policy.

However, an Association of Chief Police Officers spokesman defended the device. He told us: "This effective tool has revolutionised road policing." Mean- while, Northants patrol cars are being fitted with mobile fingerprint readers to scan drivers who are stopped.