The Wednesbury unreasonableness test is a legal standard in the United Kingdom that is designed to establish that a particular action was fully unreasonable. This test is stricter than legally establishing that an action was just unreasonable.
This unreasonableness test is usually applied to applications for judicial review featuring a public authority's decision. In order for a particular act to be Wednesbury unreasonable, it must be true that no reasonable person, who was acting in a reasonable manner at the moment in question, could have possibly performed the action. Acts which are deemed Wednesbury unreasonable are thought to be irrational.
Should an act be deemed Wednesbury unreasonable, the courts may be motivated to correct the act. Only the most extreme cases of unreasonable acts can pass the Wednesbury unreasonableness test, so very few courts have acted to correct the initial decision of the public official.
The term came from the 1948 case of Associated Provincial Picture Houses Limited versus the Wednesbury Corporation. The standard was established as a direct result of the court's decision to only correct poor administrative decisions on the grounds that they were fundamentally irrational. Lord Diplock, the judge, stated acts will only be correct when based upon "...defiance of logic or accepted moral standards that no sensible person who had applied his mind to the question to be decided could have arrived at."