A forthcoming book by Diana's one-time butler, excerpted Monday in The Daily Mirror newspaper, quoted the princess as writing that "this particular phase in my life is the most dangerous."
She reportedly wrote to Paul Burrell that someone was planning "an accident in my car, brake failure and serious head injury in order to make the path clear for Charles to marry."
The newspaper said Diana had named the person she believed was plotting against her but that it could not reveal the identity for fear of a lawsuit. It printed a photograph of part of the letter, with the name blacked out.
Royal watcher and editor-in-chief of Majesty magazine Ingrid Seward told CBS' Early Show that Diana expressed the same concerns to her.
"She said, 'But I really thought there was a conspiracy to get rid of me. I thought my car brakes were being tampered with and I had the apartment swept for bugs and of course, they found nothing,'" Steward said.
At the time, Steward said, Diana told her, "It sounds silly, doesn't it?"
"Well she told me, which is exactly what she told Paul Burrell, that she felt that she was a nuisance, that she was in the way. She never told me who she meant by the people that were getting at her," Steward says.
Diana died in an August, 1997 Paris car crash that also killed her companion, Dodi Fayed, and driver Henri Paul. A French judge has ruled that Paul's use of drugs and alcohol, and the car's high speed, caused the accident.
The new allegations are from "A Royal Duty," a forthcoming book by Burrell, the former servant whom Diana called "my rock." He was accused of stealing some of her possessions, but his trial collapsed last year after Queen Elizabeth II said he'd told her he was taking the items for safekeeping.
The Mirror quoted Burrell as saying that Diana included the allegation that someone was planning to harm her in a letter she wrote in October 1996, sealed in an envelope she marked "Paul."
He said she told him "I'm going to date this and I want you to keep it ... just in case."
Burrell said Diana believed she was regarded as a nuisance — the paper implied but did not say she meant by the royal family — once she and Prince Charles were divorced in 1996.
"She certainly felt that 'the system' didn't appreciate her work and that for as long as she was on the scene Prince Charles could never properly move on," the former butler was quoted as saying.
Burrell reportedly told the paper he had been uncertain what to do with the letter.
"That letter had been part of the burden I have carried since the princess's death," he was quoted as saying. "Knowing what to do with it has been a source of much soul-searching."
He said he hoped it would help prompt a British inquest into Diana's death, which has been the subject of a French investigation but was never officially examined in Britain.
Surrey county coroner Michael Burgess said in August that he would hold an inquest into Fayed's death, but did not set a date for it. Buckingham Palace has said there will eventually be a British investigation of Diana's death, since the law requires one, but no date has been announced.