1864 - The Style of Royal Highness, which has been in use in the United Kingdom since the Restoration period, was confirmed by Letters Patent signed by Queen Victoria. The Restoration period began in 1660, the year in which King Charles II was restored to the throne.

Victoria by the Grace of God of the United Kingdom of Great Britain and Ireland Queen Defender of the Faith To all to Whom these presents shall come Greeting : Whereas we taking into our Royal consideration that the Princes and Princesses of Our Royal Family descended from and in lineal succession to the Crown as now established by law all bear the style and title of Highness but that it has not been declared or defined by due authority what members of the Royal Family (other than the Children of the Sovereign) are entitled to the style of “Royal Highness”. We deem it therefore expedient that the same should be henceforth established defined and limited in manner hereinafter declared: Know Ye therefore that in the exercise of Our Royal and undoubted Prerogative and of Our Especial grace We do hereby declare our Royal Will and Pleasure that besides the Children of the Sovereign of these Realms the Children of the Sons of any Sovereign of Great Britain and Ireland shall have and at all times hold and enjoy the style title or attribute of “Royal Highness” with their titular dignity of Prince or Princess prefixed to their respective Christian names or with their other titles of Honor: Our Will and Pleasure further is that Our Earl Marshal of England or his Deputy for the time being do cause these our Letters Patent or the Enrolment thereof to be recorded in the College of Arms to the end that our Officers of Arms and all others may take due notice thereof. In Witness whereof We have caused these Our Letters to be made Patent. Witness Ourself at Westminster the thirtieth day of January in the twenty seventh year of Our Reign.

By Warrant under the Queen's Sign Manual.
C. Romilly

(Original letters patent, National Archives, HO 125/1)
Whitehall, Feb. 3.
The Queen has been pleased by letters patent under the Great Seal, to declare her Royal will and pleasure that, besides the children of the Sovereigns of these realms, the children of the sons of any Sovereign of Great Britain and Ireland shall have, and at all times hold and enjoy, the style, title or attribute of Royal Highnes with their titular dignity of Prince or Princess prefixed to their respective christian names, or with their other titles of honour; and further to declare her will and pleasure that the Earl Marshal of England, or his Deputy for the time being, do cause the said letters patent to be recorded in Her Majesty's College of Arms to the end that the officers of arms, and all others, may take due notice thereof.

(London Gazette Feb. 5, 1864; quoted in the Times Feb 6, 1864 p. 7D)