The American bride-to-be will gain her new regal title after the Queen is expected to grant the title of the Dukedom of Sussex on Harry when the pair tie the knot at Windsor Castle next Saturday.
Richard Fitzwilliams told Express.co.uk: “After the wedding Meghan will be known as HRH The Duchess of Sussex assuming the Queen confers the Dukedom of Sussex on Harry as expected on their royal wedding day.
“She is marrying a prince so she automatically becomes a princess.”
And when Meghan Markle, 36, exchanges her vows on May 19, Mr Fitzwilliams said she will be known as ‘Rachel Meghan’ – her first name.
However the LA born star will still be publicly known as Meghan.
Mr Fitzwilliams added: “She won’t go back to Rachel as she has always been known as Meghan.
“In the Instrument of Consent in which the Queen gives her formal written consent to the marriage she is known as Rachel Meghan.”
The Queen recently referred to Meghan by the name ‘Rachel Meghan Markle’ in a formal letter of consent approving the royal wedding to the Privy Council.
Rachel is also the first name of LA born actress Meghan’s character on Suits, paralegal Rachel Zane who is the love interest of Mike Ross, played by Patrick J.Adams.
The name of Prince William’s wife is Catherine Elizabeth Middleton, rather than Kate as she is commonly known.
Kate assumes all the feminine titles of her husband, and her full title and style is Her Royal Highness the Duchess of Cambridge, Countess of Strathearn and Lady Carrickfergus.
But Kate is not referred to as a princess because she was not born into the role.