They may exchange pleasantries when they're out together in public, but Princess Anne and Camilla, Duchess of Cornwall are far from the best of friends.
Now, decades of bitterness and jealousy is threatening to resurface as details of their love rivalry are exposed once again.
"Anne and Camilla have a lot of deep-seated drama that they'd rather the world forgot about," an insider tells Woman's Day.
"But the trouble is that it's all being dredged up – and The Crown is to blame!"
The hit Netflix drama is set to return for a third season later this year and the primary focus will be on 67-year-old Princess Anne's life, and, in turn, her love triangle with Duchess Camilla, 71.
"Anne and Camilla won't be able to pretend they don't have a storied history for much longer," the insider reveals.
"The past is getting dredged up again and it's becoming seriously awkward for the whole of the royal family, who are being forced to pick sides. It's only going to get worse as the show continues and it's especially difficult for Camilla, who has worked so hard to try and win over the public."
"This is her worst nightmare."
Anne's relationship with Camilla dates back decades before the duchess married her brother Prince Charles in 2005.
In 1970, Anne embarked on an affair with a young cavalry officer, Andrew Parker Bowles – who would later become Camilla's husband.
In fact, Camilla was seeing Andrew when he dumped her to start a romance with Anne.
And she was apparently so furious about his wandering eye that she got her sweet revenge by starting an affair of her own – with Anne's brother Charles.
"He [Andrew] was a cad, he was bonking other people, some of her friends," royal biographer Penny Junor has said.
"So when she was introduced to Charles she thought, 'Andrew is at the moment off with Princess Anne, I'll teach him a lesson.'"
While Camilla is thought to have been Charles' true love, the palace is said to have disapproved of him marrying her because she was not considered to be aristocratic enough to join the royal family, and they separated in 1973.
Meanwhile, Anne was reportedly devastated when Andrew called a halt to their romance in 1973, only to announce he was going to marry Camilla.
Shattered by the end of her romance, it's believed Anne threw herself into a rebound romance and married cavalryman Captain Mark Phillips, before later divorcing and marrying naval officer Tim Laurence.
Listen to the latest episode of the Woman's Day Uncensored Podcast, where the team discuss Simone Callahan's big interview on A Current Affair, plus they dish palace goss on Meghan Markle and lift the lid on Ita Buttrose's revenge.
But Anne's feelings for Andrew, 78, never really went away with rumours swirling for decades that they're still in love.
To this day, Andrew remains her best friend.
"The fact that Andrew left Camilla for Anne and then continued to be her friend after they got back together is something that always irked Camilla," the insider explains."Apparently, she was never happy about Andrew's relationship with Anne, who probably didn't like that Camilla was so close to Charles either."
"There's been bitterness there for a long time now and they've been famously frosty ever since."
Anne's own sibling rivalry with Charles hasn't helped relations either.
"Charles resents Anne, because he's been preparing to be King for his entire life while she's a royal rebel who does things her own way," says the insider.
"She's always been the Queen and Prince Philip's favourite, and that gets under Charles' skin."
While Camilla and Anne's icy relationship has largely gone under the radar, there have been the occasional signs that they're not exactly the best of friends.
In 2005, the Queen's private secretary drew up a document about Camilla's position in the royal family – and it declared that Anne would not have to curtsy to her.
"This was done at the behest of Princess Anne and Princess Alexandra, both of whom have given their adult lives to royal service and who saw no reason to make obeisance to this – at the time – highly unpopular woman," a former courtier told the Daily Mail.
"Princess Anne, in particular, was extremely chilly towards Camilla and made it clear she had no intention of curtsying to her ever."
And with The Crown about to serve as a painful reminder of one of the most epic royal feuds of all time.
In fact, sources say neither of them has any plans to extend an olive branch.
"Anne and Camilla have always had bad blood and this is just unearthing a lot of old wounds," adds the insider.
"Make no mistake – there's no hope of these royal rivals becoming friends!"
Want more of the best royal and celebrity gossip, including why Princess Eugenie's wedding may be called off? Pick up the latest issue of Woman's Day on stands now!