On 30 July 1948, the British Nationality Act received Royal Assent. It created citizenship of the United Kingdom and Colonies and reorganised a nationality system developed across the British Empire.

Before the Act, British nationality rested mainly on the status of British subject. The category covered people connected with Britain, its colonies and the self-governing Dominions. It reflected allegiance to the Crown rather than citizenship of one modern state.

That system became harder to maintain as Commonwealth countries gained constitutional independence. Canada created Canadian citizenship in 1947. Other Commonwealth governments were preparing laws of their own.

Britain needed to define the citizenship held by people connected with the United Kingdom and its remaining colonies. The 1948 Act created the status of citizen of the United Kingdom and Colonies, commonly shortened to CUKC.

The Act did not end the older common status. Citizens of the United Kingdom and Colonies and citizens of recognised Commonwealth countries were also British subjects. Commonwealth citizen could be used with the same legal meaning.

This allowed separate national citizenships to exist within a continuing Commonwealth relationship. A Canadian could be a Canadian citizen, while a person connected with Britain or one of its colonies could be a citizen of the United Kingdom and Colonies. Both could hold the wider status of British subject.

The Act came into force on 1 January 1949. It belonged to a period in which the Empire was becoming a Commonwealth of independent countries, while Britain still governed many colonies.

The shared status became politically disputed as migration to Britain increased. The 1948 settlement had not been designed as a detailed immigration system. However, nationality and the ability to enter Britain were closely connected in practice.

Parliament began imposing immigration controls on Commonwealth citizens through the Commonwealth Immigrants Act 1962. Later legislation placed further limits on entry and settlement. A common status that appeared workable in 1948 carried fewer practical rights as those controls developed.

The British Nationality Act 1981 replaced most of the earlier structure from January 1983. It abolished citizenship of the United Kingdom and Colonies and created separate categories, including British citizenship.

The 1948 Act recognised national independence without removing the legal connection between Commonwealth citizens. Later governments retained the separate citizenships and reduced the effect of that connection.