Patrick Jephson, Diana's former private secretary, launched the stunning accusation at Prince Charles in a new edition of his book.
He claimed Diana was "ill-prepared and untrained" to join the Royal Family and believed she had been "left to sink or swim".
He wrote: "Her private life offered no consolation, with a husband who saw her as a rival to be feared rather than a companion to be cherished.Related articles
"Charles's energy, time and attention were devoted to the older, more experienced wife of a former courtier who, like many of their secretive social circle, seemed impervious to the vulnerable princess's anguish.
"No wonder Diana suffered from chronic self-doubt, poor self-image and a persistent eating disorder."
Princess Diana's former aide, whose book is titled Shadows of a Princess, went on to take aim at the wider Royal Family.
GETTYPatrick Jephson said Charles failed to treat Diana with ‘decency’