Security agents regularly swept Buckingham Palace for hidden bugging devices, her former private secretary told the inquest into Princess Diana's death yesterday.
The checks were made to ensure conversations and phone calls by the royal family and staff remained confidential, Robert Fellowes said.
Mr Fellowes, the most senior royal aide to have given evidence to the inquest into the deaths of Diana and her lover Dodi al-Fayed, also denied allegations that he was in Paris on the night they died and that he helped arrange their "murder".
Dodi's father, Harrod's owner Mohamed al-Fayed, alleges that his son and Diana were killed in August 1997 by British security services on the orders of Prince Philip.
Mr Fayed believes Philip ordered her killing because the royal family did not want the mother of the future king to have a child with his son. He alleges that Diana's body was embalmed to cover up evidence she was expecting a baby.
Mr Fayed alleges that Mr Fellowes, who is married to Diana's sister Jane, was in Paris running the British embassy communications centre and sending messages to the secret services on the night Diana died in a high-speed car crash.
"It was being suggested that you were intimately concerned in the murder of your sister-in-law," lawyer Ian Burnett told Mr Fellowes.
Asked if he was in Paris that night, Mr Fellowes told the court "No." He said he was watching a performance at a church hall in Norfolk that evening.
Mr Fellowes, the Queen's private secretary from 1990 to 1999, revealed that Buckingham Palace rooms were regularly swept for bugs by M15 agents.
"We needed reassurance at regular intervals that there was no bugging going on," said Mr Fellowes, discussing two leaked royal phone conversations - one by Diana and one by Prince Charles. "The rooms in which business was conducted by the Queen and by her private secretaries was swept regularly."
The inquest, which has so far cost £6 million, will delve deeper into the secret world of intelligence agents when Richard Dearlove, former chief of the Secret Intelligence Service (SIS), appears on February 20th.
A Foreign Office spokesman said he and the service welcomed the opportunity to "refute the allegations of SIS involvement in the accident which led to the deaths of the Princess of Wales and Mr al-Fayed".