The aide, identified only as "Melissa," is believed to have quit suddenly, shortly after Markle married Prince Harry in May, Richard Eden reported in the publication.
"It's a real shock," a source told reporter Eden.
"Why would she want to leave such a prestigious job so soon?
"Melissa is a hugely talented person. She played a pivotal role in the success of the Royal Wedding and will be missed by everyone in the Royal Household."
The personal assistant is believed to have limited the PR damage after Markle's father Thomas was caught in a photo faking scandal before the royal wedding.
Thomas ultimately didn't walk his daughter down the aisle or attend the nuptials at all after saying he'd suffered a heart attack.
Kensington Palace won't comment on Markle's assistant's departure.
But as Radar readers know, rumors have flown about Harry kowtowing to his lady's diva demands!
In royal biographer Robert Jobson's book Charles at 70: Thoughts, Hopes and Dreams, he claimed that before the wedding, Prince Harry, 34, raised his voice and yelled, "What Meghan wants, she gets!"
Also, there are allegations that the Queen warned Harry about "difficult" Markle's attitude after a kerfuffle over what tiara she could wear on her wedding day.
Bride-to-be Markle wished to wear a tiara with emeralds but was refused after its origins could not be established.
The former Suits star, now 37, eventually wore a diamond and platinum headpiece selected by the Queen.
Markle and Prince Harry, also known as The Duke and Duchess of Sussex, announced they are expecting their first child in an October statement through Kensington Palace. Markle is reportedly due on May 2.
According to Jobson's new book, the Queen warned Harry that Markle needed to mind how she spoke to staff and to make sure she follows "family protocol."
But as Radar has exclusively reported, Markle is determined to deliver her baby using hypnobirthing methods so she can think and breathe the pain away!
"She is fixated on making the birth as natural as possible so she can boast to her L.A. pals she went drug free," a source told Radar of the former actress.
But Markle has been warned her hippie-style birthing plan may be overruled by royal protocol, which states that the baby's safety comes first.
Despite the rules, the expecting mom is "determined to do things her way," said the source.
Meanwhile, another insider told Radar that Markle's mother, Doria Ragland, has been invited to England for Christmas by the Queen, but the TV star plans to fight for her to move permanently into Kensington Palace.
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