This is also the same style that William's cousins Princesses Beatrice and Eugenie of York use.
It was earlier this year that the Queen declared that the new royal baby would be titled HRH Prince or Princess.
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A formal announcement in the London Gazette in January read, "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."
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