Angela Levin is a senior royal biographer who is known as a staunch supporter of the British monarchy.
The royal expert often criticises Prince Harry and Meghan Markle for their stepping down from their royal duties to live an independent life in the United States.
Her criticism of the Duke and Duchess of Sussex intensified after the couple sat for a tell-all interview with American TV legend Oprah Winfrey last year.
The expert on Wednesday surprised many when urged Kate Middleton, the Duchess of Cambridge, to stop trying to be someone else including Princess Diana, her late mother-in-law.
Angelia Levin said, "I think Catherine, Duchess of Cambridge should stick to being herself. She doesn't need to try to be someone else including Diana. She's great as she is."
She did not explain what made her think Kate was trying to be someone else.