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).
Related doctrines
Revising public law for an exam?
Get structured revision notes and timed SQE1 practice that build on Grounds of judicial review.
Revising law for an exam?
Get the free “50 Must-Know Cases for UK Law Exams” guide — grouped by module with the ratio for each, sent to your inbox.