Prince Charles was forced by his father to propose to the young woman he had entertained at Balmoral because the Duke of Edinburgh told him the teenager could be compromised, the book says.
Jonathan Dimbleby's book, serialised in the Sunday Times, shows how uncertain the prince was about his own feelings. As details about Diana's visits to Balmoral were interpreted as evidence that the heir to the throne had at last found the right girl, his father presented him with an ultimatum -propose marriage or end the relationship before the girl's reputation was compromised.
Diana fulfilled all the requirements to be a royal bride, says the book: she had no past; had never been in love before; and was young enough to be moulded into a future Queen. 'The Prince, in a state of emotional confusion, clung to these calculations as beacons of apparent clarity. The pressures on him began to sweep him towards his destiny,' it says.
The book details Charles and Diana's stormy relationship when they were married. It says the Princess could be violent and attempted acts of self-mutilation. Mr Dimbleby confirms incidents reported in Andrew Morton's bestseller, Diana: Her True Story.
After their honeymoon - during which the Prince first became aware of his wife's mood swings - he became aware she was jealous when he wanted other people around. She wanted to be the focus of her husband's attention but he seemed to go out of his way to avoid the moments of intimacy she craved, the book says.
'Her insecurity about his feelings for her were fed by the canker of jealousy. According to her friends, the Princess was convinced her husband was still deceiving her with Camilla Parker Bowles. According to his friends, the Princess had already reached the point of obsession. Unable to accept his word and dismissive of his protestations, she more than once exploded into a tirade of anger from which he retreated in bewilderment and despondency.'
The Princess spent long hours with the Prince's friends and advisers talking about her plight. 'The loss of freedom, the absence of a role, boredom, the emptiness in her life, the heartlessness of her husband. They listened and did their best to offer comfort and reassurance,' he writes. Mr Dimbleby claims that at times the Princess would sit hunched in a chair, her head on her knees, inconsolable. Yet, according to the author, she scoured every tabloid newspaper for photographs of herself.
The book also describes how after the birth of Prince William in 1982, the Princess was eating little and in a volatile state of mind. At this stage only one or two of the royal couple's circle of friends knew about her 'disconcerting propensity to consume large quantities of junk food' - ice cream, biscuits and popcorn.
Her bouts of despair continued and led to several suicide attempts. Mr Dimbleby writes: 'Mercifully, the cuts were always less serious than they at first appeared; they drew blood but a sticking plaster invariably sufficed to stem the bleeding.' In the autumn of 1982, Prince Charles arranged for his wife to see a psychiatrist.
The real pain felt by the Prince, the biography suggests, was his failure to stop his marriage crumbling. He was horrified by the implications for the Royal Family and the monarchy.
According to Mr Dimbleby, no one incident sparked the end of the marriage.
As life for Prince Charles became increasingly unbearable, he confided in friends that he felt powerless to save his marriage. 'Trapped in a relationship which had never penetrated beyond the everyday superficialities of their shared existence, neither could reach each other to explore the way ahead.'
The Prince of Wales, by Jonathan Dimbleby; published by Little Brown on 3 November; pounds 20.
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