"Smart royal business — or scandalous money-grubbing?" Jephson sniped, chastising the royal for her actions.
"Even back in the 1980s in the good old days of It's A Royal Knockout — when she and other Royals performed in a frankly embarrassing charity version of the game — the power of royal cypher was never really theirs. It was borrowed from the people, who allow the Windsors to reign over them."
Sarah’s royal cypher features her initial S surrounded by a coronet, which the private secretary says is another example of how the British royal family might be in "self-denial" and are claiming "material rewards for their sacrifice."
"Conspicuous consumption is bound to rouse the puritan that lurks in the most ardent monarchist."
The royal aide also pointed out how Fergie was joined by other royal family members in her extravagant spending.
"In Sarah's defence, she's not alone in helping herself to Dr Hon's generosity because — as was eventually confirmed — Zara Phillips's bank account has also benefited from his business model after she was paid £100,000 a year between August 2016 and last March."
"It's no coincidence that their self-indulgence underlies much of the criticism currently levelled at Prince Harry and his wife, with Nigel Farage just the latest (if most tactless) voice in a chorus of disapproval."
WATCH: Prince Harry and Meghan Markle locked in a battle over Frogmore Cottage renovations.
Later in the piece, the royal insider compared Fergie to his former boss, Princess Diana, describing the two as vastly different from each other.
"Diana generally had a shrewd sense of self-preservation when wading in the murky waters where royalty and money mix, taking what was offered for her good causes while carefully avoiding the more sticky inducements that often came with the package.
"Her sister-in-law somehow never quite got the knack...naively allowing her husband to negotiate a £15,000 'loan' from the disgraced billionaire and paedophile Jeffrey Epstein."
Fergie - who was the daughter of army major Ronald Ferguson and his first wife Susan Wright - has known Prince Andrew since she was a child and began her romance with the Duke at the Royal Ascot.
The Duchess of York is presently living with her ex-husband at the Royal Lodge and continues to make appearances at royal events alongside Andrew and her daughters.
For more royals news, check out this month's New Idea Royals magazine.