The United Kingdom of Great Britain and Northern Ireland uses as its national flag the royal banner locally known as the Union Flag or, popularly, Union Jack (although this name is the only one that is correct when the flag is flown on a jackstaff at sea). The current design of the Union Flag dates from the union of Ireland and Great Britain in 1801. It consists of the red cross of Saint George (patron saint of England), edged in white, superimposed on the diagonal red cross of Saint Patrick (patron saint of Ireland), which are superimposed on the Saltire of Saint Andrew (patron saint of Scotland).

Its correct proportions are 1:2. However, the version officially used by the British Army modifies the proportions to 3:5.