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The chips are produced by Texas Instruments and are claimed to have been instrumental in reducing car theft in the US as the car won't start without the recognising the chip. The transponders are said to have been fitted to some 150 million cars built by Ford, Toyota and Nissan.

In order to steal a car, according to the researchers, a criminal needs to stand next to the car user and extract the data from the key, feed it into a computer for an hour or so's number crunching and then a few minutes to break in, feed in the keycode and hotwire the engine.

The team says the ease with which this can be done is because the mathematical code used by the verification is too short and easily cracked by brute force computer power.

The team says it bought a standard microchip costing around $200 and programmed it to find the key for a petrol-purchase tag. They say that by linking 16 such chips together they cracked the key in about 15 minutes.

Texas Instruments is, understandably, denying that this is a major security hole in its system. It points out that no-one has used the Hopkins key cloning system to steal a car.