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 yesterday'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."
A Letters Patent in 1917, issued by William's great great grandfather King George V, had limited titles within the Royal Family.
It declared that "the children of any Sovereign of these Realms and the children of the sons of any such Sovereign and the eldest living son of the eldest son of the Prince of Wales shall have and at all times hold and enjoy the style title or attribute of Royal Highness with their titular dignity of Prince or Princess".
This meant that if William and Kate had a son, he would take the title His Royal Highness (HRH) and be a Prince but a daughter would only be a Lady.
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.
But now all the children that follow the Cambridges' first child can be styled Prince or Princess.
The Queen brought about the change by signing the Letters Patent before January 1 and the elaborate document transcribed in calligraphy is now kept at Buckingham Palace.
The process of granting a new Letters Patent usually takes around a month with the Crown Office often advising on the wording of the draft before the monarch puts her signature to the final copy.
A Letters Patent was granted in 1948 just before the Prince of Wales' birth because the 1917 decree excluded the children of the Queen, then Princess Elizabeth, from taking a royal style and title.
The then King, George VI, granted letters patent declaring that the children of Princess Elizabeth and Duke of Edinburgh, should "have and at all times hold and enjoy the style, title or attribute of Royal Highness and the titular dignity of Prince or Princess prefixed to their respective Christian names in addition to any other appellations and titles and honour which may belong to them hereafter".
The announcement was made on November 9, just five days before Charles was born.
The Princess Royal's children Peter and Zara Phillips, who do not hold titles, were in fact not entitled to be HRHs under the 1917 decree.
In 1996 a Letters Patent removed the title HRH from royal former wives, meaning Diana, Princess of Wales and Sarah, Duchess of York were, following their divorces, no longer entitled to use the style.
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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