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One of the common-law grounds of judicial review of administrative action. It is presumed that public authorities are never empowered to exercise their powers irrationally; therefore irrational action by a public authority is considered to be * ultra vires. Although it denotes behaviour that falls short of what is to be expected of a rational public authority, the precise parameters of the term are unclear and it has been used to describe a range of behaviour. It is often used interchangeably with the term * Wednesbury unreasonableness but has become the more common term since the case of Council of Civil Service Unions v Minister for the Civil Service [1985] AC 374 (HL), in which the terms irrationality, illegality, and procedural impropriety were used to define the common law grounds or heads of judicial review.

Subjects: Law.

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