“Men in grey” at Buckingham Palace are plotting to bring down Meghan Markle, a royal biographer has claimed.

Anna Pasternak, author of a biography of Princess Diana, has made the wild claim the Duchess of Sussex, 37, is seen as a “threat to the throne” by Palace insiders.

Pasternak, who has a book coming out on Wallis Simpson, thinks Meghan has “rejuvenated the spirit” of the American divorcee who had a relationship with Edward VIII, The Sun reports.

The then king’s relationship with Simpson led to him abdicating the throne and the couple married in 1937.

Pasternak, told the Sunday Express’S magazine: “Meghan is like a grenade that’s been thrown into this archaic system and there are explosions already going on which I think will continue”.

“In a sense Meghan has rejuvenated the spirit of Wallis and I hope people take the view of her as a kind, witty, loyal and dignified person who unfairly became the most vilified woman in the world.”

These so-called “men in grey”, who have never been named, were previously mentioned by Diana during her marriage to Prince Charles.

Pasternak added: “I fear if Meghan and Harry’s star rises too high that will be perceived as the courtiers Princess Diana called men in grey as a threat to the heir.

“She [Meghan] cannot set the agenda, she has to toe the line, and support the monarch and heir. I don’t think she realises that.”

The comments come as it was revealed Meghan and Prince Harry made a secret visit to see her mum, Doria Ragland, at her Los Angeles home.

While it is not clear when the trip was made, a neighbour told The Sun: “Meghan will always come back to Doria, this will always be her home”.

“They are forever in touch, and Meghan has been here to stay on more than one occasion since going public with Harry.

“I even saw him here once, but it’s not my place to say any more.”