The United Kingdom of Great Britain and Ireland was the formal name of the United Kingdom from 1 January 1801 until 12 April 1927. It was formed by the merger of the Kingdom of Great Britain (itself having been a merger of the Kingdoms of England and Scotland) and the Kingdom of Ireland.

Following Irish independence on 6 December 1922, when the 1921 Anglo-Irish Treaty came into effect, the name continued in official use until it was changed to the United Kingdom of Great Britain and Northern Ireland by the Royal and Parliamentary Titles Act of 1927. The part of the island of Ireland that seceded from the United Kingdom of Great Britain and Ireland in 1922 today constitutes the Republic of Ireland.