It may never be possible to establish if the princess was pregnant when she was killed in a car crash on August 31, 1997 – but she was on the contraceptive pill, the inquest jury was told. Members later saw closed circuit TV footage of Diana and Dodi Fayed at the Ritz Hotel in Paris, in the hours before their deaths.

The jury also heard from coroner Lord Justic Scott Baker that it would have to consider claims that the driver of the crashed car was not drunk when he lost control, but forensic samples had been tampered with to make him appear so, as part of the plot directed by the Duke of Edinburgh and carried out by MI6.

Dodi Fayed's father Mohamed al-Fayed believes Diana's body was illegally embalmed on the orders of MI6, the coroner said.

In France, permission to embalm must come from the deceased's family, but the decision was taken by Diana's personal secretary, Michael Gibbins, and Colin Tebbutt, a former Royal protection officer who was working for her as a security consultant.

Mr Tebbutt called Mr Gibbins from Paris and was told that the Royal undertakers were not due to arrive in the city until around 5pm on August 31, 1997, by which time Prince Charles was due to be there to view the body, which had begun to deteriorate.

The coroner said: "These men may paint a picture of finding themselves unexpectedly in a difficult situation – a decision should have been made by relatives, but who was there to call?

"To do nothing would have been, in effect, a decision not to embalm.

"You will have to consider whether, as it is claimed, there was a breach of French law in embalming Diana's body and, if there was, whether this was more than technical failure to obtain the necessary approval."

The coroner said the question of whether Diana was pregnant was relevant in two ways.

"First, her pregnancy or suspected pregnancy is said to have provided the motive or part of the motive for killing Diana.

"Second, her body was embalmed by the French and it is said that the purpose of this was to conceal that she was pregnant."

He said no pregnancy test was carried out at the hospital where she was taken that night because there appeared to be no reason to do so. A post mortem found no evidence of pregnancy.

Lord Justic Scott Baker said: "You will hear evidence concerning the extent to which a post- mortem examiner can see physical signs of early pregnancy. For obvious reasons, there would not be much to see.

"You will also hear evidence that Diana was taking a contraceptive pill."

He went on: "It is likely that pregnancy is a matter that cannot be proved one way or the other in scientific terms in this case.

"You will, of course, consider the scientific evidence such as it is but you will also hear evidence from several sources about what Diana had to say to her friends, and intimate details of her personal life." Those sources are likely to include her former butler, Paul Burrell.

It has been alleged that MI6 was monitoring the princess's calls and learned she was pregnant by this method.

The jury heard that Harrods owner Mr al-Fayed claims he found out Diana was pregnant after receiving a call on the afternoon of August 30, 1997.

The jury also heard about letters alleged to have been sent by the Duke of Edinburgh to Diana.

Lord Justice Scott Baker told the jury that Diana's then butler, Paul Burrell, had maintained these letters did exist.

However, it appeared that, if they had existed, they have gone "missing". The coroner said: "We shall have to see whether the mystery unfolds in the coming weeks."

The hearing continues.