Diana told 'without any doubt' that she was being bugged by a five-strong surveillance team

An electronic surveillance expert made the astonishing discovery after Diana asked him to check her Kensington Palace apartment for listening devices because she feared "dark forces" were snooping on her.

Former soldier Grahame Harding located a suspected bug behind a wall in her bedroom, adjacent to a room which had been used by Charles.

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It came a year after Diana had written a note to her butler Paul Burrell in which she said "my husband is planning an accident in my car."

Yesterday the inquest also heard that:

Diana thought the death of her former lover Barry Mannakee in a motorcycle crash had been "arranged deliberately".

She believed members of the security services were monitoring her movements, including listening in to her calls.

It was "common practice" for communications with the Royal Family to be monitored by the security services.

Mr Harding was introduced to Diana by the Duchess of York in 1993, a year after her separation from Charles when she settled at Kensington Palace and he remained at Highgrove.

The following year she told Mr Harding she believed listening devices had been fitted in her new quarters. He swept the apartment for bugs four times during four months in 1994.

He said: "My equipment detected an electronic signal which indicated that a possible bugging device may have been present behind a wall in her bedroom. Princess Diana was present when I found this signal.

"As you walk into her bedroom, I believe there was another room off it where His Royal Highness the Prince of Wales had a room and it was on that wall."

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He said was unable to get behind the wall to examine the device. But there was "no indication" that the fabric of the wall had been altered. He said he swept the room again later that day and the signal had gone.

"I could not give an explanation to the reading. It could have been innocent electronic equipment in another room. But the noise behind the wall was very similar to a transmitter device."

Diana never told Mr Harding who she suspected was spying on her. "She said 'there are dark forces' and never really expanded on that in any way," he said.

Mr Harding, who served with the Royal Signals before setting up a company offering protection to merchant banks, said he provided "clean" new mobile phones for Diana because she feared they were being bugged.

The inquest also heard Diana had been told by well-placed "friends" that there was a five-strong team in an "organisation" she believed the security service had instructed to conduct a surveillance operation on her.

The allegation emerged in notes made by a senior officer about a 1994 meeting between Diana and the head of the royalty and diplomatic protection department.

Colin Haywood-Trimming, a former protection officer who headed the Scotland Yard team that guarded Charles and Diana, told the inquest: "Over the years I was aware she had this feeling she was being listened to."

The inquest heard that Mr Burrell, a future witness, will assert that it was "common practice" for communications with the royals to be monitored by the security services - Mr Burrell has already made public a letter from Diana describing her fears that Charles was plotting to kill her so he could marry their sons' nanny, Tiggy Legge-Bourke.

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The note, sent in October 1993 and made public two years ago, read: "This particular phase in my life is the most dangerous.

"My husband is planning an accident in my car, brake failure or some serious head injury in order to make the path clear for him to marry Tiggy."

Mr Burrell also said that he feared for his own safety because the Queen had warned him of "dark forces" at work.

Harrods boss Mohamed Al Fayed believes Diana's death alongside his son Dodi in Paris in 1997 was a plot organised by MI6.

Before her relationship with Dodi, Diana was said to have embarrassed the Royal Family with a string of lovers that included former England rugby captain Will Carling, ex-cavalry officer James Hewitt, sports PR executive James Gilbey and art dealer Oliver Hoare.

The inquest also heard that Diana believed the death of another of her former lovers, police bodyguard Barry Mannakee, had been "organised".

Their year-long relationship ended in 1986 when he was removed from royal protection duties after Charles discovered the affair. Mr Mannakee died a year later in a motorcycle accident.

Mr Haywood-Trimming said Diana was suspicious about the death. He said: "She suggested it had been organised."

The inquest continues.