The Queen should be quizzed over conversations she allegedly had with ex-royal butler Paul Burrell, Mohamed Al Fayed's lawyers have said.
Princess Diana's sudden death shocked the nation
Mr Burrell has claimed she warned him of "powers at work in this country that we have no knowledge about".
The request came at a pre-inquest hearing into the deaths of Princess Diana and Mr Al Fayed's son, Dodi.
Coroner Lady Butler-Sloss and Michael Mansfield QC earlier argued over "late" papers and possible inquest locations.
The princess and Dodi Al Fayed died in a car crash in August 1997 in the Pont D'Alma tunnel in Paris.
Mohamed Al Fayed alleges the crash was part of a secret plot by the British establishment.
Mr Mansfield, for Mohamed Al Fayed, said of Mr Burrell's claim: "The inquiries which we suggest be made to assist is for Her Majesty being directly approached and asked was there evidence of conversations as alleged by Mr Burrell."
Mr Burrell made the claim in a collapsed Old Bailey trial for theft against him.
Lady Butler-Sloss said she would find out what the protocol would be for such a matter, saying that "whatever the protocol may be should be observed".
Earlier, there were a number of heated exchanges between Lady Butler-Sloss and Mr Mansfield.
When he said there had been "lots of promises and little action" over the "late" disclosure of documents, she said some 11,000 pages of documents and 400 photographs had now been released to interested parties.
While Mr Mansfield said some of his team's criticisms could have appeared "mildly impatient", Lady Butler-Sloss said they had been made in "not terribly polite terms".
"I have a feeling that the fact this is an inquest sometimes escapes the attention of those representing interested persons," she said.
Tactics "used in litigation often have no place in an inquest", she added.
Lady Butler-Sloss, who will stand down as coroner in June, accused Mr Mansfield of putting her "in the dock".
Lord Justice Scott Baker, one of Britain's most experienced judges, will take over as coroner for the inquests, which are expected to take place in October.
Lady Butler-Sloss announced last month she would be stepping down because she lacked the experience required to deal with an inquest with a jury.
Baroness Butler-Sloss will stand down in June
Mr Mansfield attacked her over the timing of that decision.
She had headed up the inquest for some time, "never indicating at any stage you were feeling unable to deal with such a situation", he added.
She and Mr Mansfield also clashed over news that Lord Justice Scott Baker wants to hear the full inquests in Court 73 at the Royal Courts of Justice in London.
Mr Mansfield described the venue as "totally inadequate".
Speaking at the end of the hearing, Lady Butler-Sloss said she and Mr Mansfield had "got a little heated this morning".
"I was totally wrong to do so," she added.
"I enjoyed having you here before me as I have all the others."
Lord Justice Baker will now decide the final venue for the full inquests.
In July last year, then royal coroner Michael Burgess quit the inquests, blaming a "heavy and constant" workload.
Lady Butler-Sloss, formerly president of the High Court Family Division, took on the role in September.
The hearings were originally to take place in May but were put back until October after lawyers for Mohammed Al Fayed requested more time to allow them to prepare.
This came after Harrods department store owner Mr Al Fayed won a High Court ruling that the inquests should be heard before a jury.