The Queen has declared that if the Duke and Duchess of Cambridge have a girl then she will be titled Princess.
Under past rules only a first-born boy born to William and Kate would automatically become a prince, while a daughter would have been called Lady.
The Queen has made the change by issuing new Letters Patent in a move that was widely expected because of the move for legislation to change the rules to allow women to succeed to the throne.
A notice published in Wednesday'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."
The Duchess at the BBC's Sports Personality of the Year Awards
Charles Kidd, editor of Debrett's Peerage and Baronetage, said: "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."
Similarly if the Duchess, who is believed to be just over 12 weeks pregnant, has twin boys or a second son at a later day, any second born son would be treated as the son of a Duke and would not be an HRH, but instead would have the title Lord.
A Letters Patent gave William his title following his wedding
However, the change means that all the children that follow the Cambridges' first child can be styled Prince or Princess.
The Queen issued a Letters Patent to change William's title following his wedding, granting him "and the heirs male of his body lawfully begotten the dignities of Baron Carrickfergus, Earl of Strathearn, and Duke of Cambridge".
The Earl and Countess of Wessex's children Lady Louise Windsor and Viscount Severn are entitled to be a Princess and Prince as children of the son of the Sovereign.
But the couple decided, with the Queen's agreement, that their children would use the courtesy titles as sons or daughters of an Earl rather than the style Prince or Princess
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