The policeman leading the inquiry into the death of Diana, Princess of Wales today hinted that some of the unofficial theories about the tragedy may have some validity.

Lord Stevens suggested that Mohammed Al Fayed - who claims that the Princess and his son Dodi were killed by British spies on the orders of the Duke of Edinburgh - had been "right" to raise some of his concerns about the Paris car crash.

He said in an interview that the investigation is "far more complex than any of us thought".

But Lord Stevens, the former Commissioner of the Metropolitan Police, did not specify which areas the Harrods boss had been correct to highlight.

The inquiry was ordered nearly two years ago by the Royal Coroner, Michael Burgess, amid continuing theories over the deaths, some of them rather wild.

Diana, 36, and Dodi, 42, were killed along with chauffeur Henri Paul when their Mercedes crashed into a concrete pillar in the Pont d’Alma underpass on August 31, 1997. They were being pursued by paparazzi photographers after leaving the Ritz Hotel for Mr Fayed’s apartment.

Mr Al Fayed has claimed that Diana and his son were victims of a plot orchestrated by the Duke of Edinburgh and carried out by the British intelligence services.

He says that Paul was fit and healthy, and not strung out on drink and antidepressants as the French police investigation concluded after two years.

Internet rumours allege that Paul was in the pay of MI6, and died with £100,000 in his bank account and a large sum of cash in his pocket. The amount of time taken to transfer the Princess to hospital, and the whereabouts of a Fiat car that apparently clipped the limousine, have also caused intense debate.

The emergence of a letter written by the Princess months before she died, in which she speculates that her husband, the Prince of Wales, wanted to "bump her off" in a car crash, has only added fuel to the fire.

Today Lord Stevens tantalised fans who agree that the late Princess was the victim of the British Establishment by saying that his investigation had been "worthwhile". He said: "It is right to say that some of the issues that have been raised by Mr Fayed have been right to be raised.

"We are pursing those. It is a far more complex inquiry than any of us thought."

Asked why that was so, he added: "I think it is generally the case of these things, when you actually go into them and look into them in minute detail.

"We have only got a small squad doing this and a lot of the issues have been brought up by Mr Fayed and his investigations."

Lord Stevens went on: "We have had a lot of co-operation from the French authorities, we have had the car back in this country which is being examined, and we are looking at a lot of other issues.

"We have new witnesses, we are re-examining other witnesses, and, at the end of the day, I think what people want is a thorough investigation going where the evidence takes us, and some of the answers to some of the questions that have been raised in national newspapers and in other parts of the world.

"That is the job I have been asked to do and that is the job I’ll do."

Last month it emerged that the Prince of Wales had met Lord Stevens as part of the inquiry. Lord Stevens saw the Prince at Clarence House to question him for several hours about the events that led up to the death of his former wife.

The interview is due to be broadcast on GMTV's Sunday Programme this weekend. The inquiry team's report is expected to be presented to the royal coroner early this year, ahead of an inquest into the death of the Princess.

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