Queen Elizabeth hosted the milestone bash for her eldest son, inviting senior royals and a wide range of Windsor friends and associates - but not Camilla.
"The Prince was infuriated by his mother’s continued hostility towards the woman he loved," Ms Junor wrote.
Headstrong Camilla went on to hold her own celebration in honour of the Prince on his actual birthday, November 14.
Sadly for Charles, all three of his siblings as well as Her Majesty and the Duke of Edinburgh declined Camilla's invitation.
Ms Junor also remarked how difficult it was for Prince William and Prince Harry to be "slowly introduced" to their father's new partner so soon after losing Princess Diana.
But while relations began on a shaky footing, the Duchess of Cornwall is now part of the furniture as far as the monarch and the rest of the clan are concerned!
It seems Queen Elizabeth wasn't the only one getting under Charles' skin.
WATCH this video to see what he called his brother, Prince Edward!