Princess Diana and her friend Dodi Fayed were killed

Scotland Yard Commissioner Sir John Stevens was speaking at the spot where Diana died with Dodi Al Fayed in a car crash in Paris in August 1997.

After emerging from the Pont de l'Alma tunnel, he said he intended to "draw a line" under the inquiry.

French detectives blamed the driver Henri Paul, who also died, but British police have not before been involved.

Sir John has been asked by Royal Coroner Michael Burgess to get to the truth and to nail unfounded theories.

Spies

He said: "Every single aspect of conspiracy theories and the like will be looked at by my team and the coroner."

He added: "We have got to try and do everything we can to draw a line one way or another under this inquiry. Hopefully, we can complete this by the end of December."

Sir John said he would be speaking to officers from MI5 and MI6 to investigate allegations security services were involved in the crash. And he may also speak to the Prince of Wales.

Sir John is overseeing the investigation

He travelled to Paris by Eurostar with Mr Burgess and Scotland Yard deputy assistant commissioner Alan Brown, who is second in charge on the Diana inquiry.

They visited the Ritz Hotel from where Princess Diana's Mercedes limousine set out, drove along the route the car took before it crashed, and examined the tunnel area on foot.

The road into the tunnel was closed off by French authorities and they were accompanied by Martine Monteil, who led the French investigation.

Afterwards Sir John dismissed the suggestion his visit was just a public-relations exercise as "incredibly cynical".

He will have 10 detectives working full-time on the inquiry, which may last into next year and is expected to cost up to �2m.

Sir John wanted a "feel" for the crash scene

A 6,000-page report into the tragedy produced by French judge Herve Stephan found driver Mr Paul was speeding while high on a cocktail of drink and prescription drugs.

Mr Burgess opened and adjourned his inquest into Diana's death in January.

It follows persistent claims by Dodi's father Harrods owner Mohamed Al Fayed that the couple were murdered.

BBC correspondent Caroline Wyatt said it was not possible for British officers to intervene until the French investigation finished and manslaughter prosecutions against photographers collapsed.

She said: "Sir John Stevens and Michael Burgess have been at pains to develop a close working relationship with the French police and say there is no implied criticism of their investigation."

Sir John's visit comes days after images of the dying princess were broadcast on American television and condemned by her brother Earl Spencer.