“It has long been established that if the Secretary of State misinterprets the legislation under which he purports to act, or if he takes into account matters irrelevant to his decision or refuses or fails to take account of matters relevant to his decision, or reaches a perverse decision, the court may set his decision aside” .
R (on the application of Alconbury Developments Ltd) v Secretary of State for the Environment, Transport and the Regions  2 AC 295.
A public authority must take relevant considerations into account when making decisions. The relevant considerations may be listed in legislation, or may be implied by the Court from the wording of the legislation and the context of the decision.
A public authority must not be influenced materially by irrelevant considerations. A decision that is influenced materially by an irrelevant consideration may be challenged on the grounds that it is unreasonable or that the decision-maker acted outside its powers.
Examples of relevant considerations that ought to be considered include the decision’s effect on a person’s rights, government guidance, policy statements and consultation responses.
When exercising a power given to it by legislation, a decision-maker must have regard to relevant considerations and must not be influenced materially or substantially by irrelevant matters. Failure to comply with these requirements may lead to a decision being quashed by the Court.