But they were sceptical that the 'Dianagate' tapes could have been made without more sophisticated equipment, normally only available to professional eavesdroppers, police and members of the security services.
Mobile telephone conversations can be monitored because they depend in part on radio signals. In principle, listening in is simple as tuning a radio to the Jimmy Young programme.
'Nobody in their right mind would dream of using a mobile telephone for a conversation of a sensitive or secret nature. It is a radio transmission and therefore anybody's property,' William Parsons, of Systems Elite, an anti-surveillance consultancy, said.
However the high quality of the 'Dianagate' recordings have persuaded electronics experts either that the tapes are fake or were made by professionals using equipment costing up to pounds 20,000.
According to some reports, Cyril Reenan, a retired bank manager, intercepted the conversation using a pounds 1,000 radio scanner with a 20ft aerial in the garden of his home at Abingdon, near Oxford. But to 'accidentally' stumble on the conversation, Mr Reenan would have had to be within a few miles of one of the callers.
Mobile telephone calls are made through local relay stations throughout the country, each using 160 different frequencies. They are directed through mobile telephone exchanges to land lines, then, where necessary, sent out as radio signals at the other end.
An eavesdropper would normally only be able to 'lock on' to a conversation or close to the same 'cell' as one of the telephones. Princess Diana was at Sandringham, in Norfolk, when the call was allegedly made.
In addition, casual eavesdroppers usually only hear one side of the conversation, unless local conditions are unusually good.
The quality of the recording is better than usual, they say, suggesting professional equipment was used. Highly sophisticated equipment, not normally available to the general public, can monitor mobile telephone calls from anywhere in the country.
The Celltrack, costing more than pounds 20,000, overcomes frequency changes to lock on to any call irrespective of distance or whether the 'target' telephone is moving. It is widely, but secretly, used by police and intelligence gatherers such as MI5.
The affair has spawned numerous theories that the recording, if genuine, was made by a professional looking for scandal to sell, the security services, who would probably monitor some Royal calls, or with the connivance of some factions at the Palace.
'To monitor a member of the Royal Family on a mobile telephone, all you would have to do is spend pounds 5,000 to pounds 8,000 on a good scanner and bury it in a hole in the ground outside the Palace. Then you would go back every couple of days to change the tapes,' Mr Parsons said.
'My money would not be on somebody accidentally picking it up. Somebody has gone out to make a bit of money. It could even be a member of the Royal Family. There is more to this than meets the eye.'
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