Tapes of telephone conversations include romantic and at times intimate discussions allegedly between the Prince of Wales and his friend Camilla Parker Bowles on 18 December 1989, the Princess of Wales and a man thought to be her friend, James Gilbey, 16 days later, and between the Duke and Duchess of York the next week, in which their marriage problems become apparent.
In each case, one party was on a mobile telephone and the other was on a fixed line. Until now, the use of a mobile telephone in each conversation has been blamed for the calls being picked up by third parties. Using cheap scanners, radio hams can tune in to signals broadcast from cellphones.
But yesterday, experts said that while it was possible to pick up one side of a call made to or from a mobile telephone, it was extremely difficult to record both sides of a conversation, as the words received by a cellphone and those transmitted by it were carried on different radio channels.
The electronic surveillance head of one of Britain's largest anti-espionage security firms told the Independent: 'If you wanted to get both sides of a conversation, you would need very expensive and highly sophisticated equipment; you would need to know what you were looking for and you would have to scan across hundreds of channels to find the other half of the conversation. It is virtually impossible. It would require a great deal of chance for one such conversation to be picked up, but to have three in the space of one month is simply not credible.
'The quality of the conversations, their detail and content point to recording being achieved at the fixed-link end. That would mean the fixed end had either a tape recorder receiving each end of the conversation or a transmitting device (a 'bug') sending out a signal to a recorder.'
Cellnet, which is 60 per cent owned by British Telecom, subscribed to the bugging theory yesterday. A spokesman said: 'For these listening-Toms to target and tune in to a two-sided conversation is exceptionally difficult if not impossible.
'For people determined to capture a conversation between two people, one of whom is on a mobile telephone, it would be more likely that they would tap into the fixed site because they would never know where the mobile user would be.'
After consulting a number of its engineers, the National Communications Union, which represents BT workers, said: 'There is no way these conversations can have been picked up by accident.'
A former technical officer of Personal Communication Networks, owned by Mercury, BT's rival, said: 'For some time now, people in the industry have been very sceptical about the source of these tapes. The odds of accidentally stumbling on to one are astronomical; to get three looks like a bugging operation.'
So, if it was a bugging operation, who did it and how did they release their information?
Lord Rees-Mogg, the chairman of the Broadcasting Standards Council, said he believed the Royal Family might have been targeted by MI5 during a security operation. He said the inclusion of the Duke and Duchess of York in the controversy, when attention was on the Waleses, pointed to a much wider operation.
'You would not pick up three such sensational tapes by accident,' he said. 'Three of them in such a short period suggests that there was professional surveillance taking place of the Royal Family. It certainly would not have been done by the press; it would require such skills and resources and a risk of detection quite disproportionate to any newspaper gain.
'I think the most likely explanation is that MI5 were doing it as part of a protection operation and quite innocently came across material so sensational that people who knew of it, or came to know of it, eventually leaked it.'
That would not explain why Cyril Reenan, a retired bank manager from Abingdon in Oxford, came to record the alleged conversation between the Princess of Wales and Mr Gilbey on his scanner before handing it over to the Sun newspaper, although some have suggested that the tape, featuring both sides of a conversation, was re-broadcast with the intention that it be picked up.
Perhaps the most extraordinary theory being discussed last night was the possibility that Prince Charles and the Princess of Wales might have instigated surveillance operations against each other. It was revealed on Tuesday that each had used newspapers to conduct their marital feud.
The Sun admitted yesterday that it was given information by friends of Prince Charles and faxed stories for his approval.
The Princess of Wales-Gilbey transcript appeared in the Sun. The first paper to get hold of details of the Prince Charles-Parker Bowles tape was the Daily Mirror, which is thought to have been used as a news conduit by the Princess of Wales.
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