Wednesbury Unreasonableness

Wednesbury unreasonableness is a ground of judicial review under which the High Court can quash a public body's decision as irrational. It sits within the wider framework of judicial review, which the Courts and Tribunals Judiciary describe as a type of court proceeding in which a judge reviews the lawfulness of a decision or action made by a public body, rather than a re-run of the merits of that decision.

The threshold is deliberately high. In the UK Supreme Court's judgment in Keyu v Secretary of State for Foreign and Commonwealth Affairs, the majority found the challenged decision not to hold an inquiry to be unreasonable and thus not open to challenge, while Lady Hale, dissenting, considered that the decision of the Respondents was one which no reasonable authority could reach — illustrating that even judges who disagree on the outcome apply the same demanding standard of 'no reasonable authority' rather than simply asking whether they would have decided the matter differently.

Because Wednesbury unreasonableness attacks the rationality of a decision rather than the procedure by which it was reached, it operates alongside — but is analytically distinct from — the general judicial review principle, noted by the judiciary's own guidance, that the court will not substitute what it thinks is the 'correct' decision. A claimant relying on Wednesbury unreasonableness must show the decision itself was irrational, not merely that a different, better decision was available.