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While Parliament is the primary legislating body, it is widely recognized that it does not have time to issue all legislation in the form of statute (see: Statute). Instead, statutes will grant delegated powers to other people or bodies (typically ministers) to supply the detailed legislation. The most important forms of delegated legislation are: Statutory Instruments (including Rules and Orders), Orders in Council, and by-laws.

See: Statutory instrument Order in council By-law

Because delegated legislation is primary legislation, it cannot be set aside by a court on the basis that it conflicts with case law (see: ). However, successful legal challenges are often mounted on the basis that the legislation is ultra vires -- the issuing authority acted beyond the powers conferred by statute. While it is unlikely that a Minister or a Local Authority would intentionally (or even carelessly) usurp powers in this way, it has been argued that an 'unreasonable' by-law, for example, is ultra vires. The basis of this argument is that Parliament would never intentionally delegate authority to frame oppressive or gratutious legislation.

The European communities act (1972) 'delegates' to the legislature of the European union the right to legislate for the UK. However, this is widely believed to be a different kind of delegation than the one that operates in the case of, say, Statutory Instruments. In particular, the Act gives the EU power to require the modification of UK Statutes, which is difficult to reconcile with traditional delegated legislation.