When detectives investigating Diana’s death tried to contact Philip last year, he declined to speak to them. Instead, one of his aides left a telephone answer machine message saying he did not think he could assist their inquiries.

Now Sir Miles, a 69-year-old former Gurkha officer with a gruff charm similar to that of his boss, will attempt to refute the allegations – and make sure  Philip does not have to give evidence.

Sir Miles is expected to be asked at the hearing next week about the contents and whereabouts of highly sensitive correspondence between the Duke and Princess, which Mr Al Fayed has insisted is evidence of Philip’s hostility towards Diana and his motive for murder.

Sir Miles, who joined Prince Philip’s staff in 1991, has been his private secretary since 1993 and witnessed at first hand the fall-out from the break-up of Charles and Diana’s marriage.

Simone Simmons, a friend and confidante of the Princess, claims Diana showed her a series of insulting letters written in 1992, in which Philip berated her for damaging the Royal Family with the break-up of her marriage.

She said Diana was shaken by the letters, in which her father-in-law called her a “harlot” and “trollop” and told her she should put up with his affair with the then Camilla Parker Bowles.

“They were the nastiest letters Diana had ever received,’’ she said. “She had death threats which were worded nicer than his letters. He called her a trollop and a harlot and said she was damaging the Royal Family. I thought ‘what a despicable man’ to say things like that’’.

Diana’s former butler, Paul Burrell, has confirmed the existence of letters from Philip – part of a box of highly sensitive correspondence dubbed the Crown Jewels – but denied they contained such insulting remarks.

But, whatever was in them, the letters have gone missing. Lawyers for Mr Al Fayed have suggested that they have been shredded, possibly by Diana’s sister, Lady Sarah McCorquodale, after the Princess’s death with Dodi in a Paris car crash in 1997.

The coroner, Lord Justice Scott Baker, has already told the inquest jury that it is probable the letters from Philip existed.

But at the opening of the six-month inquest 11 weeks ago, he added: “Where they went and whether they still exist remains a mystery.”