Sir Richard Dearlove, the former head of MI6, abandoned the service's customary vow of silence yesterday to deny outright that the intelligence service assassinated Diana, Princess of Wales.
The former spymaster made a highly unusual trip into the witness box to dismiss conspiracy theories over the role of secret agents in her death.
He said MI6 had no operation involving Diana or Dodi Fayed at the time of their death. He denied claims by Mohamed al-Fayed that Diana was killed by the agency and rejected as "absurd" allegations by the Harrods owner that the security services and the Duke of Edinburgh run the country.
MI6 and the other security services have for decades refused to confirm or deny any allegations about their operations.
But yesterday Sir Richard cast aside the rule because of the exceptional circumstances of the Diana inquest.
Next week, the inquest at the High Court will take evidence from a string of serving Secret Intelligence Service agents, identified only by letters, who will speak behind screens to protect their identities. Yesterday, the inquest heard Sir Richard insist it was "entirely fanciful" to suggest that the service took an interest in the Harrods owner.
Asked about its attitude to Diana, he said: "Frankly we did not take any interest in what she was doing. It's not a national security issue."
He said the service did not hold a file on the chauffeur Henri Paul, who was driving the limousine in which the Princess died in August 1997. Sir Richard also dismissed suggestions by a former agent, Richard Tomlinson, that a crash involving a blinding flash of light had been considered as an option for assassinating Slobodan Milosevic.
Ian Burnett QC, for the coroner, asked: "During the whole of your time in SIS, from 1966 to 2004, were you ever aware of the service assassinating anyone?"
Sir Richard replied: "No, I was not."
Sir Richard said it was "utterly ridiculous" to say that Prince Philip and Prince Charles were active members of MI6 and said that claims that MI6 could control a conspiracy involving the French police, emergency and forensic services was "a mischievous and fanciful allegation".
Mr Burnett told the inquest: "It is suggested that Prince Philip and the intelligence agencies really run this country and that we are not a parliamentary democracy." Sir Richard replied: "I do not want to be flippant. 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. I cannot think of any other way of saying it."