Prince William raised eyebrows in 2013 when he signed his son’s birth certificate, listing Catherine’s occupation as “Princess of the United Kingdom.”
This both irritated and confused royal watchers and accredited reporters, who were told in 2011 that the former Kate Middleton was not a princess.
“Officials at the Palace, Clarence House and in the couple’s new office at Kensington Palace had insisted Kate’s title as a Duchess meant she was not a Princess,” Richard Palmer of the Daily Express wrote in August 2013, referring to the memo he and his colleagues got after William and Catherine’s titles were announced (a gift from the sovereign).
“Kensington Palace performed a swift U-turn, insisting that, although she chose to use the title of Duchess, Kate was also Princess William of Wales,” Palmer continued, adding that a palace aide stated: “The Duchess of Cambridge would have been Her Royal Highness Princess William if she had not been given her new title, but it is not correct to say she is a Princess now.”
In a nutshell, it’s incorrect to call Catherine “Princess Catherine,” since she’s not a royal by birth. Royal Central gave a concise explanation of this rule:
If and when the title of Prince of Wales is granted to Prince William after Prince Charles becomes King, he would simply become HRH The Prince of Wales, whilst Catherine (currently Duchess of Cambridge) would adopt the title of ‘HRH The Princess of Wales’. But that would still not make her ‘Princess Catherine’ – the title of Princess of Wales is not the same as being a Princess.
For instance, even though people refer Diana, Princess of Wales ‘Princess Diana’, she never actually held that title and it was used by many as a shortened – though incorrect – version of her title. The same applies to Kate – if/when she becomes Princess of Wales.
Head over to Royal Central for the full article on what constitutes a blood royal.