Lord Stevens, the former Metropolitan police commissioner, saw the prince at Clarence House last week to question him for several hours about the events that led up to the death of his former wife.

The prince’s spokesman declined to release details of the interview, but it is known that Stevens had planned to ask Charles about his response to the bizarre allegation that he had been part of a plot to murder Diana.

Claims of the murder plot are contained in a letter by Diana stating that Charles and his friends had been plotting her death. The letter is an exhibit in the inquiry which was ordered two years ago by Michael Burgess, the royal coroner.

In her letter, full details of which were published on the day the inquest opened in January 2004, the princess wrote: “My husband is planning ‘an accident’ in my car, brake failure and serious head injury — to make the path clear for him to marry.”

Although the car crash that killed the princess has been exhaustively investigated by the French authorities, Burgess ordered his own police investigation to help to separate “fact from fiction and speculation”.

The inquiry was duty bound to confront the prince with the allegation contained in the letter which Paul Burrell, Diana’s former butler, said she wrote 10 months before her death.

A spokesman for Charles said yesterday: “Clarence House can confirm that Lord Stevens met the Prince of Wales recently as part of his inquiry into the death of the Princess of Wales. Obviously we are not going to comment in any way on the detail. But we don’t want to mislead anyone. We’ve got nothing to hide. We always said he would talk to Lord Stevens and I can confirm that that has now taken place.”

A well placed official said that there were no plans for any further interviews between Stevens and the prince.

Friends of the prince believe that the interview and inquiry will enable him to put paid to the conspiracy theories. “Hopefully this will allow Charles to put it all behind him,” said a source close to the prince.

“He and the two princes deserve to move on.”

The princess and her boyfriend Dodi Fayed, the son of Mohamed al-Fayed, the owner of Harrods, the London store, died after their chauffeur-driven Mercedes lost control as it was being pursued by photographers in a Paris road tunnel. Their driver, Henri Paul, was killed and the couple’s bodyguard, Trevor Rees-Jones, was critically injured.

A lengthy investigation by French police concluded that the death was an accident. It placed the sole blame on Paul and said he was under the influence of drink and drugs.

The 8,000-page French report has failed to satisfy the conspiracy theorists, led by Fayed. He has claimed that the princess and his son were victims of a plot orchestrated by the Duke of Edinburgh and carried out by the British intelligence services.

The Stevens team has had access to MI5 and MI6 files relating to Diana and has interviewed officers in both services. It is understood that it has established that Fayed’s claims are groundless.

Some of Diana’s friends have suggested that if the letter suspecting a plot is authentic it was written years, not months, before her death and thus cannot be taken as evidence of a conspiracy.

The prince’s interview marks the final stage of the investigation, believed to have cost more than £2.5m. Stevens’s report is expected to be finished by the spring and the inquest could be resumed.

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