Princess Diana feared the brakes of her car were going to be tampered with, 10 months before she died in a crash in Paris, her former butler has claimed.
Paul Burrell kept the letter confidential for six years
The princess allegedly wrote in a letter to Paul Burrell: "This phase in my life is the most dangerous".
She reportedly named someone who was "planning an accident in my car, brake failure and serious head injury."
The alleged letter, which Mr Burrell kept secret until now, has been published in the Daily Mirror.
The name of the alleged person has been blacked out by the newspaper for legal reasons.
Diana and her lover Dodi Al Fayed were killed early on the morning of 31 August, 1997 when
a Mercedes driven by chauffeur Henri Paul crashed in the Pont D'Alma tunnel in
Diana died in a car accident with Dodi Al Fayed and a chauffeur
In the alleged letter, Princess Diana reportedly believed the plot was "in order to make the path clear for Charles to marry".
It was reportedly written a couple of months after her divorce from Charles was finalised in October 1996.
A French inquiry in 1999 blamed Mr Paul, concluding he had taken a cocktail of
drink and drugs and was driving too fast.
In August, Surrey Coroner Michael Burgess announced he would conduct inquests into the death of Diana and Mr Fayed, but did not specify a date.
The inquests will be the first official public hearings in Britain to examine the circumstances surrounding the Princess's death.
Daily Mirror editor Piers Morgan said the letter, taken for a forthcoming book by Mr Burrell, was "sensational".
Revelations about her marriage made on Panorama programme November 1995
Refused Queen's invitation to Sandringham for Christmas 1995
Divorce finalised August 1996
Lost 'Royal Highness' title
Moved out of St James' Palace
Set up home in Kensington Palace
Allegedly wrote letter to Paul Burrell about 'car plot' in October 1996
"There has been to date, incredibly, no inquest into the death of Princess Diana and there has been no public inquiry in this country.
"Paul Burrell has watched and watched and waited and nothing has happened and he now feels that this is the time to come forward and demand those two things happen and what better way to do that than with this incredibly compelling document."
While Mr Morgan said he did not know if the letter and Diana's death were connected, he said publication of the letter was in the public interest.
"I believe at the very least the British public can expect after this morning's revelations an immediate announcement that there will be an inquest, not in another six year's time but right away, and we can finally get to the bottom of what may or may not have happened that night."
Mr Burrell was acquitted of theft in 2002. His trial collapsed after the intervention of the Queen.
He had been charged with stealing from Diana's estate.
Mr Burrell, currently in the US, said in a statement that he had reflected on events since the princess's death, and particularly since his trial collapse last year.
"During that time I have watched and listened as many individuals have claimed to know the truth about the Princess.
"I know that what was claimed to be the truth is actually far from it."
He said he decided to include details of the alleged letter in his upcoming book, A Royal Duty, because he believed "that someone has to stand in the princess' corner and fight for her now that she cannot do so herself."
Mr Al Fayed's father, Mohamed, reiterated his calls for Prime Minister Tony Blair to hold a public inquiry into the deaths.
"It is extraordinary that Paul Burrell did not volunteer this evidence in
time for the French investigation into the crash, but it is now vital that he be
called to give evidence in an independent public inquiry."
He said Mr Burrell's revelations confirmed suspicions "I have so often voiced in public and which have thus far been ignored."
"The prime minister must now accept that the time is right for a full public inquiry.
"Further delay will look as though he is colluding in a cover-up and the people of this country will not tolerate that.
BBC royal correspondent Peter Hunt said a key question remained about why Mr Burrell did not reveal the alleged letter earlier, particularly immediately after the fatal crash.
He also did not reveal it during the French investigation, nor during his court case, although Mr Burrell could argue that he intended to but was acquitted before he had a chance to enter the witness box, Peter Hunt said.
"Why didn't he reveal it during the week-long exclusive interview with the Daily Mirror last year, for which he received a considerable sum of money?
"The absence of answers to the questions will prompt the cynics to suggest he hung on to it
in order to help his book," Peter Hunt told BBC News24.