It is one of the worst-kept secrets in close royal circles that Prince Charles and his sister Anne, Princess Royal, can barely stand each other, and now the reason for their 49-year feud can finally be revealed.
While the siblings spend time together carrying out their royal duties, palace insiders tell Woman's Day that simmering beneath the false pleasantries is a longstanding feud neither party will let lie.
In fact, the rift now even threatens to break the royal family completely apart as relations between Charles and Anne hit an all time low - thanks to crafty Camilla's determination to drive an even bigger wedge between them.
"Charles and Anne are always civil with each other, especially when they're around the Queen or in public, but the reality is they're not close. There have been whispers about their ongoing feud for many years now," our insider explains.
Charles, 70 and Anne, 68, are like chalk and cheese. Anne is free-spirited and known for her love of martinis and fish and chips, while polar opposite Charles is an avid healthy eater and has a kooky obsession with having his shoelaces ironed."
"Charles is a total sticker for the rules while Anne has always been a bit of a rebel. They're total opposites and they clash because of it," says the insider who claims Camilla is at the centre of their new frostiness.
While Charles and Anne have been at odds since they were young royal rascals, their rivalry skyrocketed to new heights when they were both in their 20s, when Anne and Camilla went to war over the same man.
Anne's first boyfriend, in 1970, was Camilla's future husband Andrew Parker Bowles, who ended his relationship with Camilla at the time to start a romance with Anne.
Camilla was apparently furious and sought revenge by dating Anne's brother Charles.
Although Andrew and Camilla later reconciled and married in 1973, hostility between Anne and the Duchess of Cornwall has remained.
The Queen's private secretary even drew up a document in 2005 declaring Anne would no longer have to curtsy to Camilla.
WATCH: Prince Charles and Camilla dance during Royal Tour. Story continues after video...
"The disdain Anne still caries for Camilla doesn't help her relationship with her brother Charles, and the fact that Anne and Andrew have remained the best of friends has always gotten under Camilla's skin," another palace source explains.
But palace insiders say Camilla privately casts scorn on Anne and revels in the fact that not only did Andrew eventually choose her over Anne, but that she also cheated on him with Charles - and delights in rubbing Anne's nose in it.
Camilla's repeated flirtatious behaviour with her ex-husband infuriates Anne, who feels Camilla has only ever brought embarrassment and shame on the royal family, and heartache for Andrew.
But despite Camilla's efforts to undermine Anne, the Princess still remains the favourite in her parents' eyes.
The only daughter of the Queen and Prince Philip has always been closer to her parents than Charles, and has been allowed more freedom to live her life, which infuriates her bother.
"Charles does everything by the book and had the pressure of growing up knowing he'll most likely be king someday, while Anne has been able to have her independence and has made the most of it," the palace source adds.
When the Queen offered to make Anne's first husband - gold winning horseman Mark Phillips - an earl before they got married, they refused, allowing their Peter and Zara to grow up without the pressure of being royal.
"It's frustrating to Charles that Anne can do no wrong in the eyes of her parents. They adore her. Anne doesn't look at Charles as superior to her despite the fact that he's set to become king. To her, he's just her big brother, and that's infuriating for him," reveals our insider.
Charles is also dreading the new season of the hugely popular TV series, The Crown, focusing heavily on Anne, fearing their toxic relationship over the years may finally be exposed to the world.
"Charles and Anne keep things courteous wherever possible, but they also keep their distance, and there hasn't been any genuine warmth between them for years," our palace insider says.
"Now the world is about to be reminded of the iciness between them, which could bring it all out in the open. The only person who seems happy about it is Camilla because she feels she got the best of Anne."