Products range from simple transmitters costing less than pounds 20 to undetectable infra-red monitors at more than pounds 1,000.
The would-be eavesdropper who taped the Prince and Princess may have used professional, highly sensitive listening equipment or, as some experts suggested yesterday, a voice-activated tape-recorder, deliberately hidden by someone at Highgrove House, could even have been responsible.
'It would be the simplest thing in the world to organise a surveillance operation like this,' Richard Ludlum, managing director of Interceptor Communications in Ascot, Berkshire, said.
'You could use anything from a transmitter linked to a telephone line (which can hear conversations on the telephone or in the room) at less than pounds 100, or you could even do it without ever having entered the room by using laser equipment to detect vibrations on glass caused by people talking.'
The range of products in between includes bugs disguised as pens, calculators and executive toys for a few hundred pounds. Long-term listening can be achieved by using a bug concealed inside a power-point which feeds off the target's electricity supply - a snip at pounds 800.
Most listening devices at the bottom end of the range have two disadvantages: the short life of their power supplies and the distances over which they can broadcast to the listener, who tunes in to their pre-set frequency on an ordinary radio.
They must also be planted inside the room to be eavesdropped upon, although the person who plants them needs only a minute's access, either during a burglary or on some pretext. A favourite among surveillance experts is the Infinity Bug, which is hidden inside a telephone and takes power from the telephone's supply. 'The beauty of this device is that you can listen in from anywhere in the world,' Mr Ludlum said. 'You simply dial the person's number and when they answer you say you have a wrong number. They put the phone down, but your line is still active. By punching in a few digits, you activate the bug and listen to whatever is going on in the room.'
Such bugs can be detected by electronic 'sweeps', which identify circuitry. However, devices have been developed that can never be found because they send out an infra-red pulse from a microscopic fibre optic, rather than an aerial, although ordinary bugs can be encased in metal, the aerial must always protrude in some way, making sweeping effective; there is no such problem with the fibre optic. One security expert, who has sold infra-red equipment to the security services, said: 'The bug can be disguised as any object at all because it does not have to house an aerial. However, the listener must be close enough to see that object through the window in order for it to be fully effective.'
For a truly cloak-and-dagger operation, the smart eavesdropper will use laser equipment - costing less than pounds 1,000 - that can pick up the vibrations caused by voices on glass. The distortions in the returning laser beam are decoded into sound - without the need for anyone to gain access to plant a bug.
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