Princess Diana destroyed Prince Charles's paintings, kicked furniture and cried during a six-hour meltdown on the royal couple's honeymoon, according to an explosive new book.

The 'people's princess' is also reported to have succumbed to violent mood swings before her 1981 wedding to the next-in-line to the throne royal.

Her grandmother, Lady Fermoy, felt compelled to apologise to the Queen in 1993 for failing to warn Her Majesty about Diana's personality, which she described as "dishonest and difficult".

These revelations have been made in a new biography of Diana's famous love rival Camilla, Duchess of Cornwall, written by Penny Junor and serialised in the Daily Mail.

The book claims that Charles only became aware of his first wife's "dark side" after he had proposed to her and that mutual friends later persuaded Camilla to rekindle her affair with the Prince in order to preserve his sanity.

Charles and Diana's doomed marriage has been the subject of intense scrutiny in the UK press and around the world. The late Princess famously remarked that there had always been a "third person" in the relationship.

Now, as her 70th birthday approaches, that "third person" has told her side of the story. Camilla and and Charles celebrated their 12th wedding anniversary in April.

It started when a young Camilla Shand had a casual relationship with Charles in the 1970s, when they were both unmarried. Eventually however, she opted to take the hand of fellow aristocrat Andrew Parker-Bowles.

She called off her fling with Charles following her engagement, leaving the Prince devastated, according to the new book – smitten Charles reportedly wept and begged her not to marry Andrew but she had made up her mind.

Camilla at RAF Leeming
Camilla, Duchess of Cornwall watches a fly-past during a visit to RAF Leeming Anthony Chappel-Ross - WPA Pool / Getty Images

Unfortunately for Camilla, her husband was a serial philanderer. And so, partly in revenge and partly because she still had feelings for the still unmarried Charles, she took up with him again in 1978.

That affair would continue until Charles's engagement to Diana in 1981. The misery suffered by both as a result of that marriage could perhaps have been anticipated as Charles found himself crying on the eve of his wedding.

Prince Charles, Camilla
Britain's Prince Charles and his wife Camilla, Duchess of Cornwall arrive to visit the Vatican Apostolic Library at the Vatican REUTERS/Alessandro Bianchi

The ensuing years of adultery, torment and divorce have been well documented. But now, finally, Charles and Camilla are together.

It is tempting but ultimately fruitless to try and imagine how the Royal household might look today if Camilla had never married Andrew Parker Bowles.