Princess Diana made late night calls to her love rival Camilla threatening: “I’ve sent someone to kill you”, it has been claimed, according to Daily Mirror.
It is the latest revelation from a series of royal biographies and investigations to mark 20 years since Diana died in a limo crash in Paris.
The princes William and Harry have also been speaking about their mother and her death in detail for the first time.
Author Andrew Morton has released transcripts of taped interviews Diana secretly gave to him.
In his book, Diana was said to have been driven to despair by Charles’ long-term affair with Camilla, suffering bulimia and even attempting suicide.
But a biography of Camilla released this month, friends of the Duchess of Cornwall paint a different picture.
They say Diana was ‘unwell’ for much of the 1990s and became paranoid that those close to her were out to betray her.
Her behaviour became increasingly disturbed, it is claimed.
According to Penny Junor’s new book, she once left private secretary Patrick Jephson a note which read: “We know where you are, and so does your wife [sic]. I know you’re being disloyal to me.”
And Diana also made a number of “unnerving” phone calls to Camilla in the middle of the night, it is claimed.
“Without saying who was calling, she’d typically say: ‘I’ve sent someone to kill you. They’re outside in the garden. Look out of the window; can you see them?’ ” the book claims.
In 1994, Charles was interviewed for a BBC documentary and publicly admitted his adultery with Camilla for the first time.
It prompted her husband Andrew Parker Bowles to file for divorce.
The pair were said to have been well-aware of each other’s marital affairs but had stayed together for the sake of their children.
A year later, Princess Diana gave another equally explosive interview in which she spoke about the end of her marriage to Charles.
She famously said: “There were three of us in this marriage, so it was a bit crowded.”
In 1996, Charles and Diana agreed a settlement on their divorce thought to be worth more than £17million.
But on the advice of lawyers, Charles also issued a statement saying he had “no intention of remarrying.”
According to Ms Junor’s book, Camilla felt humiliated.
“…she felt she was being publicly consigned to a box labelled ‘mistress’,” she writes.
The pair continued to see each other in secret.
But a year later the world was rocked by Diana’s death in the Paris underpass.