Grounds of judicial review
The classic grounds on which courts review the lawfulness of a public body's decision are illegality, irrationality and procedural impropriety, as set out by Lord Diplock in the GCHQ case — with proportionality increasingly recognised as a further ground.
Last reviewed 14 June 2026
In Council of Civil Service Unions v Minister for the Civil Service (the GCHQ case) [1985] AC 374, Lord Diplock grouped the grounds as illegality (acting beyond legal powers or misdirecting in law), irrationality, and procedural impropriety (breach of natural justice or statutory procedure).
Irrationality means “Wednesbury unreasonableness” — a decision so unreasonable that no reasonable authority could have made it (Associated Provincial Picture Houses Ltd v Wednesbury Corp [1948] 1 KB 223). Proportionality is a more intensive ground, applied especially in human-rights contexts (R (Daly) v SSHD).
Key cases
- Council of Civil Service Unions v Minister for the Civil Service (GCHQ) [1985] AC 374
- Associated Provincial Picture Houses Ltd v Wednesbury Corp [1948] 1 KB 223
- R (Daly) v Secretary of State for the Home Department [2001] UKHL 26
Frequently asked questions
What are the grounds of judicial review?
Illegality, irrationality (Wednesbury unreasonableness) and procedural impropriety (the GCHQ case), with proportionality as a developing further ground.
What is Wednesbury unreasonableness?
A decision so unreasonable that no reasonable public body could have reached it — the classic test for irrationality (Associated Provincial Picture Houses v Wednesbury).