Illegality as a ground of judicial review: ultra vires, errors of law/fact, improper purpose, fettering discretion, delegation, relevant considerations, statutory/common law limits, prerogative powers
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Overview of the illegality ground
Illegality stands as the first ground of judicial review identified by Lord Diplock in his well-known analysis in Council of Civil Service Unions v Minister for the Civil Service. He described three categories for controlling administrative action by judicial review, the initial category being “illegality”, by which he meant that a decision-maker must correctly apprehend the law governing the scope of his power and act in conformity with it. A challenge on the illegality ground alleges that a public law decision-maker has acted unlawfully by exceeding its powers, or by misunderstanding them, or by otherwise misusing them. Lord Diplock’s term ‘illegality’ is helpful as a label for this ground because such a claim essentially maintains that a decision is unlawful for want of a proper legal foundation, or because its legal basis is inadequate or defective, or is in some other way legally erroneous. This is to be contrasted with claims that:
- a decision departs from rationality or reasonableness in a way only indirectly linked to its merits (ie claims within the unreasonableness or irrationality ground)
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Web page updated on 21/05/2026
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