Titulus Regius (the "Title of King" in Latin) is a famous act of the English Parliament, issued in early 1484, by which the title of King of England was given to Richard III of England.
It is an official declaration that describes why the Parliament had found (the year before) that the marriage of Edward IV of England to Elizabeth Woodville had been invalid, why consequently their children were illegitimate (and, therefore, debarred from the throne), why in the end Richard III was the rightful king.
The act was repealed by the first parliament of the new king, Henry VII of England, who had ordered all copies of it (and all related documents) to be destroyed without being read. So well were his orders carried out that only one copy has ever been found: It was included in the Croyland Chronicle, where it was discovered by Sir George Buck during the reign of James I.
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