Meghan Markle was labelled "that minx" as well as "ungrateful" by Camilla, Duchess of Cornwall, a royal biographer has claimed.
Royal expert Tom Bower has suggested the two Duchesses never got on.
The Duchess of Cornwall (left) the Duchess of Sussex at a garden party at Buckingham Palace in London which she is attending as her first royal engagement after being married.
Dominic Lipinski
Camilla, next in line to be Queen, was allegedly "suspicious" of Ms Markle.
Mr Bower also claimed that the Duchess of Cornwall, 74, was "tight-lipped" regarding her opinion of Meghan when Prince Harry brought her into the Royal family.
Mr Bower also believes that the Sussexes' silence over the announcement that Camilla would be known as Queen Consort when Prince Charles becomes King was "deliberate" and an "insult" to the family.
"Perhaps the Sussexes’ deafening silence is Meghan’s way of declaring war," Mr Bower said.
"It is believed that she has never liked Camilla, and I am sure the feelings are reciprocated.
"From the outset, Camilla was suspicious about the adventuress from Los Angeles.
He went on: "She found it hard to believe that Meghan would sacrifice her career and independence to serve silently as a team player devoted to the monarchy.
"But, as in all her conduct, Camilla remained tight-lipped. While watching Charles painfully believe that Harry was intent on publicly humiliating him, Camilla undoubtedly provided the comfort and good advice to reassure her husband that he had done the best he could.
"She could comfort him that while his authority is challenged by the Sussex’s, he would emerge victorious.
"During those inevitably endless, tortured and inconclusive conversations, Camilla is the sort who would refer to Meghan as ‘that minx’ — the self-seeking trouble- maker whose antics will always end in tears."
The Sun reported that Her Majesty, 95, said in Accession Day address that it was her "sincere wish" that Camilla be known as Queen Consort.
Camilla became Princess Consort in 2005.
