BRITAIN'S most senior policeman vowed yesterday to draw a line once and for all under the mystery surrounding the death of Diana, Princess of Wales.

Scotland Yard Commissioner Sir John Stevens, speaking at the spot where Diana died in a car crash in Paris, said he would be willing to interview the Prince of Wales as part of his inquiry into her death.

He also said he would be speaking to officers from MI5 and MI6 to investigate conspiracy theories alleging the involvement of the security services in the crash.

Sir John, who was visiting the crash scene at the Pont de l'Alma tunnel for the first time, said the underpass was much smaller and narrower than he had thought and the gradient of the road into it was steeper.

He will have ten detectives working full-time on the inquiry, which may last into next year and is expected to cost up to £2m.

The inquiry began at the request of royal coroner Michael Burgess, who opened and adjourned an inquest in January - six-and-a-half years after Diana, her then boyfriend Dodi Fayed, and driver Henri Paul died in their Mercedes limousine.

Sir John travelled to Paris with Mr Burgess and Scotland Yard deputy assistant commissioner Alan Brown.

The road into the tunnel was closed by gendarmes and Sir John, Mr Burgess and Mr Brown were guided into the tunnel by Martine Monteil, director of the Brigade Criminale, who led the French investigation.

After emerging from the tunnel, Sir John said: "There are a lot of conspiracy theories relating to the deaths of the three people in this tunnel.

"It is my job to report to the coroner every single aspect being investigated in terms of conspiracy theories.

"This is a very intricate investigation. Every single aspect of conspiracy theories and the like will be looked at by my team."