The following principles were stated in the judgment of Lord Greene, the Master of the Rolls, in Associated Provincial Picture Houses Ltd v Wednesbury Corporation [1948] 1 KB 223:
It is true the discretion must be exercised reasonably. Now what does that mean? Lawyers familiar with the phraseology commonly used in relation to exercise of statutory discretions often use the word "unreasonable" in a rather comprehensive sense. It has frequently been used and is frequently used as a general description of the things that must not be done. For instance, a person entrusted with a discretion must, so to speak, direct himself properly in law. He must call his own attention to the matters which he is bound to consider. He must exclude from his consideration matters which are irrelevant to what he has to consider. If he does not obey those rules, he may truly be said, and often is said, to be acting "unreasonably."Read more here:http://www.caselawquotes.net/U/Unreasonable_Wednesbury_Test.html
