Lord Stevens, the former Metropolitan police commissioner interviewed John Scarlett, the head of MI6, and Eliza Manningham-Buller, the MI5 director-general, last month.

He is trying to establish whether the intelligence services had any knowledge of the circumstances leading up to Diana’s death. Her lover, Dodi Fayed, the son of Harrods tycoon Mohamed al-Fayed, and Henri Paul, their chauffeur, also died in the crash.

Detectives from Operation Paget, the codename given to the £2m inquiry, have also interviewed two middle-ranking MI6 officers who were based undercover at the British embassy in Paris at the time. Both are understood to have denied any involvement.

Before she died Diana claimed in a letter that Prince Charles planned to kill her, possibly in a car crash. Paul Burrell, Diana’s former butler, suggested in a book that she feared the brakes of one of her cars would be tampered with.

Conspiracy theorists, including Fayed, have suggested that MI6 agents murdered the princess in order to prevent Dodi becoming stepfather to Prince William, the second in line to the throne.

The former renegade spies David Shayler and Richard Tomlinson have also suggested that intelligence officials may have had some involvement. Tomlinson claimed Henri Paul was a paid informant of MI6.

A police source said last week that Scarlett and Manningham-Buller had opened any intelligence files that may have a bearing on Diana to the inquiry. “It’s unprecedented. They have given Stevens complete and unfettered access,” he said.

Stevens has also interviewed Sir Richard Dearlove, who was head of MI6 from 1999 until Scarlett took over last summer. Sir David Spedding, the MI6 chief at the time, is now dead.

A lengthy inquiry by the French police concluded that the tragedy was an accident. Paul was drunk, under the influence of medication and driving too fast.

However, last year the royal coroner, Michael Burgess, announced a new inquiry. He believed that only a thorough investigation by British police would finally lay to rest some of the conspiracy theories.

There has been speculation that Paul was being used by British intelligence to keep a discreet eye on Diana as her affair with Dodi continued to attract world attention.

As a hotel security chief for the Paris Ritz, where Diana and Dodi dined on the night they died, he earned just over £20,000 a year. But French police discovered that he lived a luxury bachelor lifestyle which included taking expensive private flying lessons.

He had amassed a small fortune of five times his annual salary in 13 bank accounts. Much of the money is said to have been deposited in the months immediately before the crash.

The remains of the Mercedes car in the crash are due to be shipped to Britain this month.

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