In an interview with Yahoo's The Royal Box, royal biographer Anna Pasternak revealed the Duchess of Sussex is "heading for a tricky situation" and predicted her marriage to Prince Harry will become "increasingly difficult" if she fails to amend her A-list attitude.

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Ms Pasternak said she believes the survival of the British monarchy depends on all attention being on the heir to the throne, and the author thinks Meghan's determined and domineering unique style - similar to "the people's princess" Diana - is "not appropriate and not going to work long-term".

"Meghan doesn't understand that she cannot be this international star on a global stage - tat is the monarch - The Queen, then Charles, then William and Kate.

"They have to be the stars, that's what ensures the continuity of the monarchy," she said, per The Sun.

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Ms Pasternak also voiced concern about the future of Meghan's relationship with Harry.

"Nobody wants to cast ill on a new marriage with a young baby involved but I think it's going to become increasingly difficult."

The writer's caution comes one month after George Clooney attacked media treatment of the heavily pregnant Duchess who he claimed is being "pursued and vilified" in a similar way to the Princess of Wales.

Clooney, 57, said history was repeating itself in the case of Meghan and Diana, warning “we’ve seen how that ends”.

WATCH: George Clooney Comes to Meghan Markle's Defense, Comparing Her Plight to Princess Diana's