AS conspiracy theories go, the alleged plot to murder Diana, Princess of Wales tops them all.

The suggestion goes that Diana did not die by accident when her car crashed in Paris, but that her death was planned by dark forces who tampered with the brakes.

The release by Diana's former butler Paul Burrell of a letter in which Diana predicts a car crash assassination plot ten months before her death has given added momentum to the rumours.

The question is: Why did Mr Burrell wait six years to make the extraordinary letter public through the Daily Mirror?

He told the paper: "I believe the people of Britain and the wider world who love the Princess deserve to know the truth about her life."

He adds that he firmly believes that someone has to stand in the Princess's corner and fight for her now that she cannot do so herself.

We agree with Mr Burrell that it has taken far too long to arrange an inquest into Diana's death. Six years on, there is still no date for a hearing and that is unacceptable.

But the truth is that, no matter how thorough the inquest is when it finally happens, it will not put a stop to the conspiracy theories. If it concludes that Diana did die in an accident, it will be seen as another part of an establishment cover-up.

And it is impossible not to question Mr Burrell's motives. If he believes so fervently that the truth must be told, why didn't he make official investigators aware of the letter at the time of the tragedy?

After all, Diana has not been able to fight her own corner for the past six years, so why take up the cudgels now?

The publication of Diana's letter, which dominated the first ten pages of yesterday's Daily Mirror - relegating news of the Prime Minister's heart scare to Page 11 - comes a week before Mr Burrell's book goes on sale.

Suggestions that book sales are Mr Burrell's real motive would, of course, be just another conspiracy theory.