LONDON — A judge in Britain ruled yesterday that she alone would determine what caused the deaths of Princess Diana and her boyfriend, rejecting arguments that a jury was the best way to ensure justice.In the ruling, Baroness Elizabeth Butler-Sloss said a jury — unlike a judge — would be forbidden under British law from considering the conspiracy theories that have dogged the investigation into the August 31, 1997, deaths of Diana and Dodi Fayed.

“In particular, the jury would not be able to answer questions on allegations that a person, group or organisation had been guilty of criminal activities in respect of the death of the princess or Dodi,” she said.

Mohamed al Fayed, the father Dodi Fayed, has accused the queen’s husband, Prince Philip, of orchestrating a plot to kill Diana and Fayed, who died in a car crash in Paris. Philip has never commented on the accusation.

Late last year, a sweeping British police inquiry dismissed allegations that the princess was the victim of a murder conspiracy. The inquiry, headed by Lord Stevens, the former chief of the Metropolitan Police, said the chauffeur in the 1997 crash was drunk and speeding to elude pursuing photographers. Stevens’ report largely confirmed previous findings by French investigators.

The judge said a jury would find it difficult to cope with the volume and detail of the evidence. The inquest will delve into technical matters on the crash, creating a video simulation and expert testimony.

Over 30 witness are expected to testify, including some by video-link from Paris.

“It would undoubtedly be easier for a professional judge to carry out the necessary investigation,” Butler-Sloss wrote.

Al Fayed’s legal team had pressed the judge to call a jury, saying it was the only way the public would be satisfied that proper care was taken over the issues surrounding the crash.

“The best way to answer conspiracy theories is to actually do it in a good old fashioned court and to hear the witnesses and cross-examine them,” al Fayed’s spokesman, Michael Cole, said during preliminary hearings last week.

It is likely the inquest will begin in early May, Butler-Sloss said at the earlier hearing. Under British law, an inquest must be held when someone dies violently, unexpectedly, or of unknown causes.

The two-year French investigation, three-year Metropolitan Police inquiry and repeated legal action by al Fayed have delayed the inquest by nearly 10 years.

Diana, 36, and Fayed, 42, were killed along with chauffeur Henri Paul when their Mercedes crashed in the Pont d’Alma tunnel. The only survivor, bodyguard Trevor Rees — formerly known as Rees-Jones — was badly hurt.