British nationality law is the law of the United Kingdom that concerns citizenship and other categories of British nationality. The law is complex because of the United Kingdom's former status as an imperial power.

English law and Scots law have always distinguished between the Monarch's subjects and aliens. Until 1914 British nationality law was uncodified. The British Nationality and Status of Aliens Act 1914 codified existing common law and statute, with a few minor changes.

With the development of the modern British Commonwealth of Nations in the 20th century, some deemed the single Imperial status of British subject as increasingly inadequate to deal with the realities of a Commonwealth with independent member states. In 1948, the Commonwealth Heads of Government agreed that each member would adopt a national citizenship, but that the existing status of the British subject would continue as a common status held by all Commonwealth citizens.

The British Nationality Act 1948 established the status of Citizen of the United Kingdom and Colonies (CUKC), the national citizenship of the United Kingdom and those places that were still British colonies on 1 January 1949, when the 1948 Act came into force. However, until the early 1960s there was little difference, if any, in United Kingdom law between the rights of CUKCs and other British subjects, all of whom had the right at any time to enter and live in the United Kingdom.