Short answer
A UK court can ordinarily quash a public decision, delegated instrument or other measure when its maker exceeded legal power, misunderstood the law, pursued an improper purpose, ignored a mandatory consideration, acted with procedural unfairness or reached an irrational result. Proportionality applies where a right or statutory regime requires it; it has not simply replaced ordinary common-law review in every case.
The object challenged matters. Courts do not normally quash a Westminster Act. They may invalidate devolved legislation outside statutory competence and delegated legislation outside the authority granted by its parent Act. A policy is not unlawful merely because it could be applied unlawfully: the question is whether it authorises, directs or creates an unacceptable risk of unlawful action under the governing legal test. An individual decision is reviewed on its own facts and procedure.
Judicial review is supervisory, not a merits appeal. Even after illegality is proved, relief is discretionary and causation matters. A court may refuse relief where the same outcome was highly likely, remit rather than substitute a decision, or suspend or limit a quashing order's retrospective effect. Claimants must therefore identify the precise act, legal power, ground, evidence, deadline and useful remedy at the outset.
First classify the measure and the decision-maker
A label such as “legislation” conceals several different rules:
- Westminster primary legislation: courts interpret it and may declare incompatibility under the Human Rights Act 1998, but do not ordinarily quash it.
- Devolved primary legislation: an Act of the Scottish Parliament, Senedd Act or Northern Ireland Assembly Act is invalid so far as it exceeds competence under the relevant devolution statute.
- Delegated legislation: regulations, orders and rules must stay within the power and purpose conferred by the parent Act and comply with required procedure.
- Policy or guidance: usually structures future discretion and may be reviewed without waiting for every individual application.
- Individual decision: legality depends on the statutory scheme, reasons, evidence, procedure and impact on the claimant.
England and Wales generally use the Administrative Court and CPR Part 54. Scotland and Northern Ireland have separate court procedures; standing, deadlines and remedies are not identical.
Illegality asks whether the public body stayed within its authority
Illegality includes acting without power, misconstruing a statute, improper purpose, fettering discretion, irrelevant considerations, omitting mandatory ones or unlawful delegation. A qualifying material error of fact may also suffice.
Delegated legislation receives no immunity because Parliament approved it through an affirmative procedure. In , the Supreme Court held that a proposed residence test for civil legal aid fell outside the power conferred by the Legal Aid, Sentencing and Punishment of Offenders Act 2012. The question was the scope of the enabling Act, not whether the policy seemed desirable.
Public bodies may adopt policies for consistent decision-making, but must remain willing to consider exceptional cases where the statutory discretion requires it. A purportedly non-binding document can be reviewable if officials or affected people are expected to treat it as authoritative.
Policies are tested by what they direct, not every imaginable misapplication
In , the Supreme Court restated the common-law test for challenging policy content. A policy is unlawful where it positively authorises or approves unlawful conduct, or where the authority has a duty to provide accurate advice and the policy misstates the law. It is not enough that an official might apply lawful guidance wrongly.
The Court cautioned against treating a policy as if it were legislation:
“Policies are not to be read as if they were statutes.”
—
Guidance may still be unlawful if statute requires it to explain a safeguard, it fetters discretion, or its inevitable operation is unlawful. A challenge should identify the wording and duty it defeats.
Procedural fairness depends on context and practical opportunity
Fairness may require notice of the case, disclosure of decisive material, a meaningful chance to respond, an unbiased decision-maker, consultation or reasons. The content is variable: a licence revocation, disciplinary finding, benefit decision and national-security assessment do not demand identical procedures.
Where consultation is legally required—by statute, promise or established practice—it must occur while proposals remain genuinely open, give sufficient reasons to permit an intelligent response, allow adequate time and conscientiously consider the responses. In , failure to explain realistic alternatives made a council-tax consultation unfair in its statutory context.
A clear representation or settled practice may create a legitimate expectation. The court weighs its terms, reliance, statutory duties and overriding public interest, but cannot require unlawful action.
Apparent bias is assessed objectively. Reasons need not answer every submission, but must be sufficient where statute, fairness or the nature of the decision requires the affected person to understand why they lost and whether a challenge is realistic.
Irrationality is supervisory; proportionality is structured
Traditional irrationality review asks whether a decision lies outside the range lawfully open to the decision-maker. The intensity varies with context. Courts scrutinise threats to fundamental rights, access to justice or the constitutional order closely, while allowing institutional latitude on expert evidence, resource allocation and political policy.
The court checks for a rational evidential basis; it does not choose its preferred evidence. In , substantial evidence showed a real refoulement risk under the proposed Rwanda arrangements. The result turned on evidence and legal obligations, not abstract policy disagreement.
Proportionality normally asks whether a measure pursues a legitimate aim, is rationally connected to it, uses no less intrusive equally effective means and strikes a fair balance. It is required in Human Rights Act cases and certain statutory or constitutional contexts. In ordinary common-law review, did not establish proportionality as a universal replacement for irrationality.
Devolved legislation can be invalid; Westminster legislation normally cannot
Devolution statutes define legislative competence through subject-matter reservations, territorial connection, Convention rights and other limits. A court can declare a devolved provision outside competence and grant the remedy authorised by the settlement. It may use powers to suspend the effect of its decision so the legislature can correct the defect.
In , the Supreme Court rejected ordinary irrationality review of an Act of the Scottish Parliament within competence. Lord Hope explained:
“The Scottish Parliament takes its place under our constitutional arrangements as a self-standing democratically elected legislature.”
—
That does not remove statutory competence review. It distinguishes review of legislative validity from treating a democratic legislature like an administrative decision-maker.
Procedure can determine the case before the merits do
In England and Wales, requires permission and sufficient interest. requires filing promptly and normally within three months after grounds arise; planning and procurement regimes have shorter deadlines. A pre-action letter does not stop time.
The claimant should normally exhaust a suitable alternative remedy. At permission, the claim must be realistically arguable. The public authority owes candour about relevant facts and documents.
Section 31 requires refusal of relief where it is highly likely the outcome would not have been substantially different absent the defect, subject to exceptional public interest. Delay, prejudice to third parties and good administration may also defeat relief. Standing and remedial provisions differ in Scotland and Northern Ireland, so an England-and-Wales form should not be copied across jurisdictions.
Quashing is one remedy, and its effect can be adjusted
A quashing order nullifies the impugned decision or instrument to the extent ordered; the usual next step is reconsideration according to law. The court rarely substitutes its own merits decision, although section 31 permits substitution for a court or tribunal error where only one lawful result was possible.
, inserted by the Judicial Review and Courts Act 2022, permits a quashing order to be suspended and its retrospective effect removed or limited. The court considers justice, good administration, affected persons, reliance and the defect's nature. Other remedies include prohibiting and mandatory orders, declarations, injunctions and interim relief. Damages require a separate legal basis; judicial review unlawfulness alone does not automatically generate compensation.
Conclusion
A viable judicial-review claim aligns five things: the correct reviewable measure, the decision-maker's legal authority, a recognised ground, admissible evidence and a useful remedy filed in time. Westminster Acts, devolved Acts, regulations, policies and individual decisions cannot be placed in one remedial box. Illegality and fairness remain central; irrationality respects the boundary between supervision and merits; proportionality applies where the governing rights or statute call for it. Proving an error is only part of the task, because permission, alternative remedies, causation and the court's control over quashing may decide what practical relief follows.
Sources
This article provides general legal information, not advice for a specific matter. Laws, facts and procedural deadlines may change the result. Follow the linked primary sources and obtain local advice before acting.