Under the law of the United Kingdom, high treason is the crime of disloyalty to the Crown. Offences constituting high treason include plotting the murder of the sovereign; committing adultery with the sovereign's consort, with the sovereign's eldest unmarried daughter, or with the wife of the heir to the throne; levying war against the sovereign and adhering to the sovereign's enemies, giving them aid or comfort; and attempting to undermine the lawfully established line of succession. Several other crimes have historically been categorised as high treason, including counterfeiting money and being a Catholic priest.

The Crown of Ireland Act 1542 is an Act that was passed by the Parliament of Ireland on 18 June 1542, which created the title of "King of Ireland" for monarchs of England and their successors; previous monarchs had ruled Ireland as Lords of Ireland. The first monarch to hold the title was King Henry VIII of England.