The Secret Intelligence Service (SIS) normally neither confirms nor denies stories about its operations in the shadowy world of espionage glamourised in the James Bond movies.
But former MI6 boss Richard Dearlove decided to speak out over the deaths of Diana and her lover Dodi al-Fayed in a high-speed Paris car crash in 1997.
Dodi's father, luxury storeowner Mohamed al-Fayed, says that his son and Diana were killed by British security services on the orders of Prince Philip.
Fayed has said her killing was ordered because the royal family did not want the mother of the future king having a child with his son, and that Diana's body was embalmed to cover up evidence she was expecting a baby.
That produced a forceful denial from Dearlove at the inquest into the deaths of Diana and Dodi, where al-Fayed has made a string of allegations against British Establishment figures.
Lawyer Ian Burnett asked Dearlove: "During the whole of your time in SIS, from 1966 to 2004, were you ever aware of the service assassinating anyone?"
"No, I was not," he told the court, rebutting what he called a "very personal allegation" over Diana's death.
Burnett, putting a string of al-Fayed's allegations to Dearlove, said: "It is suggested that Prince Philip and the intelligence agencies really run this country and that we are not a parliamentary democracy."
"I do not want to be flippant," Dearlove said. "I'm tempted to say I'm flattered, but this is such an absurd allegation that it is difficult to deal with ... It's completely off the map."
Dearlove dismissed as "utterly ridiculous" claims that Philip and Prince Charles were members of MI6.
He said MI6 needed the authorisation from the Foreign Minister to carry out any operation which broke the law.
Asked to confirm that no authorisation was sought over Princess Diana, he said: "I can absolutely confirm that."
Next week three secret service officers are scheduled to appear at the inquest but their identities will not be revealed.
The court will be cleared of the media and public when they give evidence, which will be piped by audio link to an annex.
(Editing by Dominic Evans)