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Under UK law, the term 'alien' is defined by exclusion: any individual who is neither a British citizen, nor a member of any one of several non-citizen 'privileged' groups in the UK, is considered an alien. The term 'alien' itself is ordinarily used to refer to a foreign national present in the UK. On the whole, the law assumes that aliens are subject to the same legal rights and responsibilities as citizens of the UK, but with the following exceptions: (i) Non-EU aliens have no right to vote in UK elections or become MPs. (ii) Non-EU aliens have no right to work, except where a permit has been properly granted. (iii) Non-EU aliens must register their whereabouts with the police during prolonged stays in the UK.

Any alien who engages in a criminal act may be deported.

See also British citizenship.