Mr Lacey says: “I actually feel quite sorry for Camilla.
"Just as Diana was caught by antiquated values, so Camilla was also trapped.”
Prince Charles and Camilla had been engaged in a passionate romance in the seventies, however, their relationship was forced to end because of one backward royal rule: the heir to the throne could not marry a woman who had been in previous relationships.
In contrast to Camilla, the virginal figure of 19-year-old Lady Diana Spencer was seen as the perfect bride for the future King of England, and according to Express.co.uk, the match was encouraged by the royal family.
Mr Lacy continues: “Charles meets Diana - but right from the beginning there’s the shadow of Camilla there all the time.”
The royal historian also analysed the now famous photograph of Princess Diana walking next to her future husband's mistress at the Ludlow races in 1980.
Mr Lacey says: “It’s a very sad photograph.
“You look at that picture, and you think, ‘What is going through the minds of those two women?’
“Camilla, already Charles’ mistress, and the girl beside her who is going to discover.
“When you see that, I certainly feel pity for Diana, because she’s about to get involved in a big game that she didn’t know half the details of at that stage.”