Overview of the illegality ground

Illegality is the first of the grounds on judicial review set out by Lord Diplock in his classic exposition in Council of Civil Service Unions v Minister for the Civil Service:

‘one can conveniently classify under three heads the grounds upon which administrative action is subject to control by judicial review. The first ground I would call “illegality”...

By “illegality” as a ground for judicial review I mean that the decision-maker must understand correctly the law that regulates his decision-making power and must give effect to it.’

Judicial review on the illegality ground is a claim that a public law decision-maker has acted unlawfully by exceeding its legal powers, or misunderstanding or in some way abusing them. Lord Diplock’s label ‘illegality’ seems useful for referring to this ground of judicial review because a claim based on it essentially argues that a decision is unlawful because it has no proper legal basis, or an inadequate or defective legal basis, or is otherwise legally flawed.

This contrasts with claims that:

1. •

   a decision