Three initial observations can be made about the Supreme Court’s analytical approach to prerogative powers in the Miller judgment.
First, the Court’s judgment has clarified the proper framework for analysing prerogative powers. After establishing that a relevant prerogative power exists (see  and ), the next stage is to determine the extent of that prerogative power. This comes before any possible questions of abrogation by statutes arise. Thus, the Court delimited prerogative powers generally, and the foreign relations treaty prerogative specifically, in holding that such executive powers cannot affect domestic law or domestic rights (see  – ). As such, the third theoretic step (exclusion by statute i.e. the abrogation line of authority such as de Keyser’s) and the fourth step (control of the exercise of such prerogative power) became unnecessary once the Court found that domestic law and rights would be affected if and when Article 50 were triggered (see  – ). As the Court stated “rather than the Secretary of State’ being able to rely on the absence in the 1972 Act of any exclusion of the prerogative power to withdraw from the EU Treaties, the proper analysis is that, unless that Act positively created such a power in relation to those Treaties, it does not exist” ().The Court could not find such positive authorisation in any of the relevant statutes from 1972 to 2015.
Second, the Supreme Court upheld the Divisional Court’s two-pronged reasoning that prerogative power does not extend to either changing domestic law or affecting domestic rights (see ). This may have significant consequences for the use of prerogative powers in the international sphere where domestic or acquired rights would be affected, and is of potentially wider application than if the Court had confined its analysis to circumstances where acts on the international stage result in changes to the (domestic) constitutional framework. It may, combined with the findings in relation to the principle of legality (), have implications for what the authorisation bill needs to cover.
Third, we learn that the “frustration of purposes” line of authority (e.g. FBU) falls within the remit of extent analysis (that is a red-line parameter around what prerogative powers can ever do), rather than as a matter of exercise control (that is as one factor within discretionary control of using prerogative powers such as abusive or irrationality or proportionality control): see . This, it seems to me, is more stringent than Laker, FBU and the Divisional Court judgment in Miller could be read as. What we do not learn, however, is the interrelationship between the Bill of Rights tests of dispense/suspend (which, it seems, falls within the extent analysis stage) and the exclusion stage (that is abrogation) tests. This is unfortunate, and could readily herald a more stringent approach by expanding the extent analysis box (i.e. extending inherent limitations on the prerogative) and thus reducing the scope for getting to the exclusion-by-statute analysis.
Jack Williams is a Barrister at Monckton Chambers. He was instructed in the Miller litigation at both Divisional Court and Supreme Court levels by Bindmans LLP on behalf of the Pigney Respondents/Interested Parties (known as “The People’s Challenge”). All views are strictly his own and do not necessary represent those of either his clients or the rest of Monckton Chambers.
(Suggested citation: J. Williams, ‘The Supreme Court’s Approach to Prerogative Powers in Miller: An Analysis of Four E’s’, U.K. Const. L. Blog (25th Jan 2017) (available at https://ukconstitutionallaw.org/)