Secondary legislation, also known as delegated legislation or subordinate legislation, is law that is not enacted by a legislative assembly such as the UK Parliament, but made by a government minister, a delegated person or an authorized body under powers given to them by an Act of Parliament. Secondary legislation is dependent on its parent act, which prescribes its parameters and procedures. Statutory instruments are the most frequently used type of secondary legislation, with approximately 3,500 made each year in the UK, although only about 1,000 need to be considered by Parliament. Secondary legislation is used for a wide variety of purposes such as fixing the date on which an Act of Parliament will come into force, setting fees for a public service, or establishing the details of an Act of Parliament. Secondary legislation can make small changes to an Act, and the Act must say what changes can be made to it by secondary legislation and what process the secondary legislation will follow. Secondary legislation can also create new rules or add more details to an Act.

Secondary legislation is necessary in cases where specific duties or rights are to be applied to civil servants, as terms or conditions of their service, or existing terms or conditions amplified or restricted, as these alter the legal relationship of the civil servant with his or her employer created by the Civil Service Act. The power to make secondary legislation may be conferred directly by the Civil Service Act, which may indicate which bodies have this power and what matters may be regulated under it, and it may prescribe specific limits within which the power must be exercised.