Prince Charles only married Princess Diana because his father sent him a threatening letter telling him to propose because her "reputation was on the line".

Charles panicked when he got the note from Prince Philip amid fears in the royal family that his indecision had "impugned" her honour, reports the Daily Mail.

He thought that Philip, who communicated with his son by letter and not by speaking to him, was issuing him an order that said: "You've got to get engaged".

READ MORE: • Letters reveal what Diana did on her honeymoon

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According to family friend Pamela Hicks, Charles "wasn't in love, he wasn't ready ... psychologically he assumed his father bullied him, so he read it as a bullying letter".

The extraordinary claim is in a new biography of Charles by New York Times best-selling author and royal family biographer Sally Bedell Smith.

She chronicles the life of the heir to the throne in a riveting book, Prince Charles: The Passions and Paradoxes of an Improbable Life, which will be released on April 4.

She details how Charles and Diana were a terrible match from the start of their ill-fated union in July 1981.

According to claims in the book, Diana was prone to breaking down in public. Photo / Getty
According to claims in the book, Diana was prone to breaking down in public. Photo / Getty

Charles only married Diana, a former kindergarten teacher, because his real love Camilla Shand, now his second wife, Camilla, Duchess of Cornwall, was not virginal enough for royal custom.

During their rows at Kensington Palace Diana used to taunt Charles that he would "never be king", knowing it caused him deep consternation.

Bedell Smith claims that Diana used to hit Charles over the head when he knelt down to pray by their bed at night and continue berating him during his prayers.

The book portrays Charles, 68, in an unflattering light and writes that he and his staff "dealt with Diana's bewildering and often infuriating behaviour by placating her, trying to distract her and, ultimately, out of frustration, abandoning her".

The book makes it clear that the real love of Charles' life is Camilla, 69, whom he first met in 1972.

Even though Camilla married another man, Andrew Parker Bowles, her affair with Charles would continue for much of his marriage to Diana. Photo / Getty
Even though Camilla married another man, Andrew Parker Bowles, her affair with Charles would continue for much of his marriage to Diana. Photo / Getty

Camilla was an upper class girl and the daughter of a British Army officer with a low, husky voice.

Lady Annabel Goldsmith called her "an intensely warm maternal laughing creature, with enormous sex appeal".

Charles adored how Camilla "always listened" to him, Bedell Smith writes, and in her he
"found the warmth that he yearned for".

But it wasn't to be because Charles was not ready to settle down with her at the age of 24.

More importantly, family friend Patricia Mountbatten said, there were "obvious problems" with Camilla.

The conventions of the time called for a princess to be virginal, or at least appear so, and Camilla "had a history", as Mountbatten put it, "and you didn't want a past that hung about".

Even though Camilla married another man, Andrew Parker Bowles, her affair with Charles would continue for much of his marriage to Diana.

By the time Charles reached the age of 30, the pressure to get married was intense and after a series of flings, Diana entered his life.

According to Bedell Smith she was in the wrong place at the wrong time and she married the wrong man.

In 1980 Charles' relationship with Diana, the daughter of John Spencer, Viscount Althorp, had become "clandestine".

With no proposal from Charles, the royal family felt her "honour had been impugned" and that she was "besieged" by the press which camped outside her front door.

Charles was told to either propose to Diana or release her. Photo / Getty
Charles was told to either propose to Diana or release her. Photo / Getty

At the time Diana, who was just 19 and lacked the confidence that came so easily to Camilla, confessed to a neighbour she felt "miserable".

In January 1981 Diana arrived for a three-day visit to Sandringham, the royal family estate in Norfolk, England, and Bedell Smith says it was a "tense month all round".

At this point Philip decided to weigh in with his letter in which Bedell Smith writes that he "told his son that Diana's reputation was on the line because of all the speculation in the press".

Philip said: "Charles should either propose to her or release her. In either event, he should make a decision shortly."

Bedell Smith writes: "Perhaps Charles could have understood the nuances of his father's message more clearly if he and Philip had talked it through.

"But written communications were the regrettable norm for father and son. Charles chose to interpret the letter as coercive and accusatory".

Another factor was the restrictions imposed on Charles' choice of a bride.

According to Bedell Smith: "In 1980, more than a decade after the sexual revolution had started, he was hemmed in by the royal custom of marrying a virgin, or at least a woman who seemed virginal.

"He was forced, in effect, to rob the cradle ... the 12-year gap between Charles and Diana
was essentially unbridgeable.

"They had no intellectual connections, few mutual friends, no interests in common and none of the shared life experiences he would have had with a contemporary.

"Although Camilla had the same limited upper-class education she was on Charles wavelength, absorbed as he was by hunting and other country pursuits, at home in the same social circle, in a way Diana could never be".

The marriage would be marked from the start by jealousy from Diana about Camilla which fuelled her bulimia, self-harm and paranoia.

One of the saddest aspects Diana's short and tragic life was the failure of those around her, friends and family alike, to convince her to get a proper diagnosis and treat her extreme symptoms of mental instability: bulimia, self-mutilation, depression, acute anxiety, paranoia.

Diana felt the royal family was cold and did not welcome her. Bedell Smith says her relationship with The Queen was "correct" but the Monarch's "self-containment unnerved" Diana.

Diana never felt welcomed by the queen. Photo / Getty
Diana never felt welcomed by the queen. Photo / Getty

Diana's suspicion over Camilla soon spilled over into a poisonous obsession the played to her insecurities, Bedell Smith writes.

The book says: "She was tormented by feelings of emptiness and detachment, she feared abandonment, she had difficulty sustaining relationships and she kept those closest to her on tenterhooks with her sudden moods swings, explosive rages and long sulks".

Diana banished most of Charles' friends from their lives because she was convinced they were conspiring against her, the book says.

She even got rid of Charles' Labrador, which she made him keep with an aide.

One time Diana heard Charles murmuring to Camilla affectionately on the phone and on another occasion she found a bracelet with the initials ''GF'' engraved on it, or ''Girl Friday'', his nickname for her.

Diana used to break down in public; she burst into tears a week before the wedding at a polo match.

The book says that Charles lacked the knowledge or the temperament to genuinely help her.

He tried to find her a therapist but she required "constant support" and he could not give it to her.

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