Queen Elizabeth II has formally declared that the future daughter of the Duke and Duchess of Cambridge will be a Princess.

With a long-awaited royal baby due in late spring or early summer, the Queen today issued what is known as a Letters Patent to announce that Prince William and Kate Middleton’s baby would become an HRH – Her Royal Highness – even if she is a girl.

In doing so the Queen rectified a century-old anomaly that meant a royal daughter would not automatically become a princess.

A decree made by King George V in 1917 stipulated that that while a son would become a prince, a daughter would not become a princess.

King George had ordered that the titles HRH and Prince and Princess should be restricted to the children of the sovereign, the children of the sovereign’s sons and the eldest son of the eldest of the Prince of Wales.

Royal sources said the Queen felt it was time such a long-standing imbalance was formally rectified.

The Queen’s proclamation, issued through the House of Lords on December 31 last year but only made public today reads: “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 honor.”

Queen Elizabeth II has formally declared that the future daughter of the Duke and Duchess of Cambridge will be a Princess

While the announcement is no great surprise, it is, nevertheless, an important milestone for the future third-in-line-to-the-throne.

Letters Patent are a type of legal instrument in the form of a published written order issued by the monarch. Their name derives from the Latin pateo, meaning exposed and accessible.

They are a form of public proclamation and a rare exercise of extra-parliamentary power by the sovereign, known as royal prerogative.

Prior to the establishment of parliament the sovereign ruled absolutely by the issuing of his or her personal written orders.

That said, such documents – which are rare nowadays – tend to be issued with informal government approval.

The Letters Patent would have been marked with the Great Seal of the Realm, the chief seal of the Crown, used to show the monarch’s approval of important State Documents.

Any document relating to a close member of the Royal Family requires the use of a unique blue seal.

Buckingham Palace refused to comment on the timing of the announcement and whether it meant that Kate Middleton was now safely past the crucial 12-week stage of pregnancy.

However well-placed royal sources suggested that the Queen had been keen to do for some time.

Kate Middleton, who celebrated her 31st birthday yesterday, was taken into hospital in early December suffering from acute pregnancy-related sickness and has retained a low profile ever since.

Last month a new law was published – the Succession to the Crown Bill – which end the system of male primogeniture, meaning the Duke and Duchess of Cambridge’s first child will become monarch whether they are a boy or a girl.