The royal sister will be a princess thanks to the Queen
Her Royal Highness The Duchess of Cambridge was safely delivered of a daughter at 8.34am.— Kensington Palace (@KensingtonRoyal) May 2, 2015
SUNDAY: The Duke and Duchess of Cambridge's daughter will be officially titled a princess thanks to an intervention by the Queen.
The couple have taken their youngest child after Catherine gave birth following a two-hour labour, but they are yet to announce a name.
Under a decree made almost 100 years ago, titles were limited by King George V to mean that only the eldest son of the eldest son of the Prince of Wales could be styled Royal Highness.
The Letters Patent meant that although her older brother is HRH Prince George of Cambridge, William and Kate's new baby would have been Lady Mountbatten-Windsor.
That rule was revoked by Queen Elizabeth ahead of the birth of Prince George in 2013, when she declared that "all the children of the eldest son of the Prince of Wales should have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".
Congratulations to the Duke and Duchess of Cambridge on the birth of a daughter. Margie & I wish the young Princess a long and happy life.— Tony Abbott (@TonyAbbottMHR) May 2, 2015
It has been a quarter of century since a baby born to the British royals has taken the title of princess, following the birth of William's cousin, Princess Eugenie, in 1990.
In later life, when William becomes king, the new princess may be given the honorary style of Princess Royal, as Princess Anne does now.
It is customarily given by the monarch to his or her eldest daughter.
Day to day, the newest addition to the royal family will be known as the Princess of Cambridge and will be fourth in line to the throne.
Her name has not yet been announced, although bookmakers' favourites include Charlotte and Alice. Royal supporters celebrate outside St Mary's hospital in west London Royal supporters celebrate outside St Mary's hospital in west London
According to the royal family's official website: "For the most part, members of the Royal Family who are entitled to the style and dignity of HRH Prince or Princess do not need a surname, but if at any time any of them do need a surname (such as upon marriage), that surname is Mountbatten-Windsor."
Prince William uses Wales as his professional surname in the armed forces, as does his brother Prince Harry.
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The Duke of Edinburgh wanted the royal family to use his name, Mountbatten, when the Queen came to the throne in 1952 and she incorporated it into the name for their direct descendants in 1960.
"I am nothing but a bloody amoeba," Prince Philip once complained to a friend. "I am the only man in the country not allowed to give his name to his own children."The lengthy titles and styles of the royal family can be tricky to navigate but the monarchy's website assures subjects that there are "no obligatory codes of behaviour" when meeting the Queen or her relatives.
For anyone wanting to stick to tradition, the correct formal address for the monarch is "your majesty" and then "ma'am".
Other members of the royal family should be addressed as "your royal highness", then "sir" for men and "ma'am" for women.
Additional reporting by PA
8.26PM SAT: IT'S a girl!
The new princess was born on Saturday at 8:34AM at London's St Mary's Hospital shortly after it was announced the Duchess had gone into hospital ahead of the birth.
Early reports suggest the duchess spent two hours in labour.
The yet unnamed baby girl will be fourth in line to the throne - behind her grandfather Prince Charles, father Prince William and her brother Prince George.
The baby's birth has seen Prince Harry, who is currently in Australia, relegated to fifth in line to the throne.
Clarence House once again broke with royal formality to announce the birth of their second child via social media.
The young couple continued with a tradition they set three years ago after they shared news of the baby's birth to the world on the internet.
It was released in stark contrast to the usual tradition of posting a signed announcement on an easel outside Buckingham Palace.
News of the birth of the couple's first child, Prince George, was tweeted to the world as well as appearing on the British Monarchy's Facebook page when he was born on July 22, 2013.
British royal correspondents covering the event were given momentary notice of the baby's birth so they could break the story online and on television ahead of the official tweet.
All attention now turns to what the young couple will name their new bundle of joy.
The royal baby name will adhere to protocol, which dictates a name previously used by British royalty, as well as one with positive connotations.
Topping the list with bookmakers for names the new royal baby girl may be called include Alice, Charlotte, Elizabeth, Victoria, Diana, Alexandra, Grace and Mary.
Before news of the imminent arrival of the baby itself, minor dramas outside the Lindo Wing included two giant teddy bears being delivered and interest in some rather unusual paintings featuring the Queen and the Duchess of Cambridge.
The Duchess, who was a week overdue, is being looked after by consultant obstetrician Guy Thorpe-Beeston, who is the surgeon-gynaecologist to the household.
He was to be joined in the delivery room by Alan Farthing, the Queen's surgeon-gynaecologist.
Twitter Feed for the royal baby: