Duke and Duchess of Cambridge
If William and Kate have a daughter she will be a princess (Reuters)

If Kate Middleton and Prince William's royal baby is a girl, it will be a princess, the Queen has declared.

Britain's head of state has said that all children of the Duke of Cambridge should be given the title of prince and princess.

Previously, only the first-born son of the Prince of Wales would automatically receive a royal title, becoming a prince. If the Prince and his partner had a girl, she previously received the title Lady.

The Queen made the change by issuing new Letters Patent, which allow her to give orders without involving parliament.

Published in the London Gazette, the formal announcement said: "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."

Change of ancient law

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."

This announcement comes as the government works to change legislation so that if William and Kate have a daughter, she will become Queen.

Ancient law currently in place means a male child would take precedence over a girl, despite order of birth.

The Duchess of Cambridge yesterday celebrated her 31st birthday. It is thought Kate, who is believed to be 12 weeks pregnant, spent the day privately with her husband at their home in North Wales, where William works as an RAF Search and Rescue pilot.

Her last public appearance was at the BBC Sports Personality of the Year Awards on 16 December.