From next month, many of the leading manufacturers will include a new style of 'coded' central-locking unit in their cars, which they hope will foil thieves who have shown they can easily get round present systems.
The latest device police have uncovered is known as a 'grabber' and is about the size of a Filofax. It records the signal used to lock or unlock a car, then recreates the same signal so someone can play this back to the car and undo the locks.
The devices are simple boxes consisting of little more than an oscilloscope and signal generator - both standard pieces of electronic equipment. They will operate at up to about 200 yards - far enough for someone to use sitting in another car in a car park.
Earlier versions of grabbers did not usually record the signal, but tried out different frequencies in rapid succession until they happened to scan past the one that worked. Manufacturers got round this by altering their locking units so that they required their coded signal to be held for two seconds - this meant it took too long for a potential thief to test out each frequency at random. But this does not deal with the new recording grabbers.
The Metropolitan Police said it first came across the newer devices about six months ago. They are available through mail-order magazines and are said to be on sale in some south London pubs. Police claim they are not in widespread use in Britain, but car manufacturers are sufficiently anxious to change their locking systems.
Conventional remote central locking systems send out either an infra-red or a radio signal consisting of a string of dots and dashes, rather like Morse code. This code is unique to each car, although it will always be at a frequency set within a narrow band approved by the Government. The code is usually the same for locking and for unlocking the car.
The way round the new grabbing devices is for the locking system to change its code every time it is used. This way the potential thief records a code that is already out of action by the time he tries to play it back to the car. This is the basis of the systems going into cars from next month.
Both the unit inside the car and the handheld unit will be pre-programmed to change codes at random - although in step with each other - each time the system is used, flipping through many thousands of possible codes.
One alarm maker, which supplies nine large car companies, will offer the new 'rolling code' locking systems soon and is negotiating a deal with a leading insurance company to offer customers discounts of up to 25 per cent if they have the new system fitted.
The alarm company said clients included Mercedes, Volvo, Ford, Rolls-Royce and Toyota. Its system shuffles through more than 4 billion codes at random, but could add up to pounds 200 to the cost of a car.
Alan Beaumont, who handles marketing liaison for Vauxhall in Europe, said most car manufacturers were moving in this direction: 'If they are not, they are making a big mistake,' he said.
Det Chief Insp Jim Reynolds, of the stolen vehicle squad of the Metropolitan Police, said he was aware of the devices, but that there was no evidence as yet that they were being bought or used. 'We have alerted the car trade and many of them have started doing things to tackle it.' He pointed out the device had a legal use, to help 'repo men' to get into cars they are entitled to repossess.
Remote central locking systems have proved problematic in the past. Those that use infra-red signals are particularly prone to open or lock inadvertently. One source said it was common knowledge in the car industry that television remote control units would unlock some vehicles. This should be more difficult with the latest generation of locking systems.
Only a tiny percentage of the cars have remote central locking systems, but this is expected to rise over the next few years.