English law and Scots law have always distinguished between the Monarch's subjects and aliens, but British nationality law was uncodified until the British Nationality and Status of Aliens Act 1914 codified existing common law and statute, with a few minor changes.
Some thought the single Imperial status of "British subject" was becoming increasingly inadequate to deal with a Commonwealth of independent member states. In 1948, the Commonwealth Heads of Government agreed that each member would adopt a national citizenship (Canada had already done so), but that the existing status of British subject would continue as a common status held by all Commonwealth citizens.
The British Nationality Act 1948 marked the first time that married British women gained independent nationality, regardless of the citizenship of their spouses. It established the status of Citizen of the United Kingdom and Colonies (CUKC), the national citizenship of the United Kingdom and colonies on 1 January 1949. Until the early 1960s there was little difference, if any, in UK law between the rights of CUKCs and other British subjects, all of whom had the right at any time to enter, live and work in the UK.
Independence acts, passed when the remaining colonies were granted independence, contained nationality provisions. In general, these provisions withdrew CUKC status from anyone who became a citizen of the newly independent country, unless the person had a connection with the UK or a remaining colony (e.g. through birth in the UK). Exceptions were sometimes made in cases where the colonies did not become independent (notable cases include the Crown Colony of Penang and the Crown Colony of Malacca, which were made part of the Federation of Malaya in 1957; CUKC status was not withdrawn from CUKCs from Penang and Malacca even though they automatically acquired Malayan citizenship at the time of independence).
Between 1962 and 1971, as a result of fears about increasing immigration by Commonwealth citizens, the UK gradually tightened controls on immigration by British subjects from other parts of the Commonwealth, which included CUKCs without familial or residential ties to the UK. The Immigration Act 1971 introduced the concept of patriality, by which only British subjects (i.e. CUKCs and Commonwealth citizens) with sufficiently strong links to the British Islands (e.g. being born in the islands or having a parent or a grandparent who was born there) had right of abode, meaning they were exempt from immigration control and had the right to enter, live and work in the islands. The act, therefore, had de facto created two types of CUKCs: those with right of abode in the UK, and those without right of abode in the UK (who might or might not have right of abode in a Crown colony or another country). Despite differences in immigration status being created, there was no de jure difference between the two in a nationality context, as the 1948 Act still specified one tier of citizenship throughout the UK and its colonies. This changed in 1983, when the 1948 Act was replaced by a multi-tier nationality system.
The current principal British nationality law in force, since 1 January 1983, is the British Nationality Act 1981, which established the system of multiple categories of British nationality. To date, six tiers were created: British citizens, British Overseas Territories citizens, British Overseas citizens, British Nationals (Overseas), British subjects, and British protected persons. Only British citizens and certain Commonwealth citizens have the automatic right of abode in the UK, with the latter holding residual rights they had prior to 1983.
Aside from different categories of a nationality, the 1981 Act also ceased to recognise Commonwealth citizens as British subjects. There remain only two categories of people who are still British subjects: those (formerly known as British subjects without citizenship) who acquired British nationality through a connection with former British India, and those connected with the Republic of Ireland before 1949 who have made a declaration to retain British nationality. British subjects connected with former British India lose British nationality if they acquire another citizenship.
In spite of the fact that the 1981 Act repealed most of the provisions of the 1948 Act and the nationality clauses in subsequent independence acts, the acquisition of new categories of British nationality created by the 1981 Act was often dependent on nationality status prior to 1 January 1983 (the date the 1981 Act entered into force), so many of the provisions of the 1948 Act and subsequent independence acts are still relevant. Not taking this into account might lead to the erroneous conclusion, for example, that the 1981 Act's repeal of the nationality clauses in the Kenya Independence Act of 1963 restored British nationality to those who lost their CUKC status as a result of Kenya's independence in 1963. This is one of the reasons for the complexity of British nationality law; in complicated cases, determining British nationality status requires an examination of several nationality acts in their original form.