Wednesbury [1948]

Lord
Greene MR gave two meanings to reasonableness.
‘Umbrella
sense’ –
Allowed
courts to intervene where decision was of type which could not be made at all –
therefore illegal.
Unreasonableness
used synonym for host of other more specific grounds of attack, eg acting for
improper purposes, acting male fide.
‘Substantive
sense’ – Decision may be attacked if conclusion reached is ‘so unreasonable
that no reasonable authority could ever have come to it’.