'It was an enormous row like nothing ever before witnessed within palace walls,' adds the royal source.
With just weeks to go until Princess Eugenie's wedding to businessman Jack Brooksbank, the Queen invited Eugenie's mother Fergie to join the family during their annual retreat to Scotland.
'She wanted everyone to get along, but extending the royal olive branch has turned into a total disaster,' says the insider.
'Her Majesty has put aside differences with Prince Andrew's ex-wife and expected everyone else to do the same.
'She had no idea tensions that have simmered between Fergie and Camilla would boil over into a very un-royal public brawl.'
According to GLOBE's sources, Fergie has been 'tormented for decades' by Camilla's affair with Prince Charles.
'She's previously told pals, 'I'll never forgive her'', a royal source tells.
‘It seems that Fergie has never really confronted Camilla over the pain and heartache she caused Diana over her affair with Charles,’ explains the royal source.
‘Fergie and Diana were close friends and she saw first-hand the anxiety and depression that [Camilla and Charles’] affair brought to her – and it’s always churned away inside Fergie.’
Despite Diana and Fergie falling out towards the end of the princess’ life, the pair were extremely close at one point, a fact which the Duchess of York has never forgotten.
‘Diana was one of the wits I knew – nobody made me laugh like her,’ she gushed in an interview with Harper’s Bazaar.
In fact, Princess Diana was instrumental in orchestrating the romance between Sarah Ferguson and Prince Andrew, with Diana taking the chance to matchmake, by inviting her future sister-in-law to a party at Windsor Castle back in 1985.
So the fact that Fergie would want to defend Diana over Camilla comes as no surprise.
Just one year after Diana had set up Fergie and Andrew, Prince Charles began secretly seeing Camilla behind his unsuspecting young wife’s back.
Diana was also busy looking after their two young sons.
And now, in the run up to the 21st anniversary of Diana’s tragic death, Fergie has finally let rip at Camilla.
‘Fergie couldn’t hold it in any more and really let Camilla have it during after dinner drinks at Balmoral,’ continues the source.
‘She and Camilla have never seen eye-to-eye, but being at Balmoral brought back a lot of memories of Diana and all the time they spent there together – so it was only a matter of time before she let Camilla have it.’
‘They really went at it, Fergie didn’t hold back and told Camilla she had Diana’s blood on her hands for all the pain she caused – no-one really knew where to look, it was a fight that no-one’s seen before within palace walls.’