The Queen has declared that if the Duke and Duchess of Cambridge have a daughter she will be titled Princess.
Under past rules a girl born to William and Kate would have been styled Lady and not known as Her Royal Highness - only a first-born boy would automatically become a Prince.
But the Queen has taken action, by issuing new Letters Patent, to insure her great grandchild has a title suitable for a future monarch.
Charles Kidd, editor of Debrett's Peerage and Baronetage, said the alteration was expected, especially in light of moves to pass legislation removing discrimination surrounding women succeeding to the throne.
"It seemed a logical presumption that it would be amended," Mr Kidd said.
He added: "Letters Patent are simply a way that the Sovereign signifies making an alteration or proclamation that doesn't go through Parliament.
"It's not very rare. They're quite commonly used for example whenever a life peer is created.
"When Prince William was made Duke of Cambridge that was a new Letters Patent under the Great Seal of the Realm."
A notice published in Tuesday's London Gazette announced the decision: "The Queen has been pleased by Letters Patent under the Great Seal of the Realm dated 31 December 2012 to declare that all the children of the eldest son of the Prince of Wales should have and enjoy the style, title and attribute of royal highness with the titular dignity of Prince or Princess prefixed to their Christian names or with such other titles of honour."