Mohamed al Fayed's controversial theories about the death of Diana, Princess of Wales, came under fire yesterday.

The Harrods tycoon listened quietly as layers of his theory that Diana was murdered in a 1997 car crash by MI6 on the order of the Duke of Edinburgh because she was pregnant and set to get engaged to his son Dodi, were stripped down.

John Macnamara, a retired Metropolitan Police detective chief superintendent and Mr al Fayed's director of security in August 1997, accepted he had lied in public when he claimed in a television interview that driver Henri Paul had only drunk pineapple juice.

The jury also heard Lord Stevens, who carried out Operation Paget, the official investigation into the conspiracy theories, publicly denounce for the first time "scurrilous allegations" about his professionalism.

He condemned suggestions that he or his team had been negligent, not done their job properly and that he had been "got at" regarding the evidence in the report.

Calling the allegations "quite outrageous," he said: "I will take that on my behalf, but I will not have it said about people who worked for me for four years, who sometimes cannot defend themselves on these issues."

Lord Steven's report, published in December 2006, found the deaths were a tragic accident and also that Mr Paul was three times the French drink-drive limit.

In contrast to an eye-catching headline back in 1997 which claimed Mr Paul was "drunk as a pig", Lord Stevens described him on the night as "under the influence of alcohol".

He told the jury at the Diana inquest at the High Court in London: "Looking at the CCTV, looking at the witness statements, we knew that Henri Paul by account had a high tolerance for drink and, in all honesty, we could not say he was drunk, in our definition."

Lord Stevens claimed Mr Paul "appeared to have an alcohol problem and obviously had a very high tolerance to alcohol".

Lord Stevens also hit back at the suggestion he had used a November 2006 meeting with Mr Paul's parents, Jean and Gisele, to deliberately mislead them over what he would say about how much their son had drunk.

He said: "That's outrageous, and I'm looking for an apology in relation to that."

The hearing was adjourned to Monday when Mr al Fayed is set to give evidence.