Does Wightman mean that Miller was decided incorrectly?
The Court of Justice of the European Union has today in C-621/18 Wightman held that an Article 50 notice is unilaterally revocable, subject to specified conditions. (For a summary, see here .) It will be recalled that the Supreme Court in Miller  UKSC 5 assumed, on an invitation from the parties, that Article 50 was unilaterally irrevocable. Does that mean that Miller was wrongly decided?
In my view, the answer to this is an emphatic “no”. This follows for three reasons.
First, the Supreme Court did not decide that Article 50 was unilaterally irrevocable. It merely stated that it was “content to proceed on the basis” that Article 50 was not unilaterally revocable, but “without expressing any view of our own” on the point: see para. 26. It would therefore be wrong to claim that Miller is directly contradicted by the CJEU’s judgment in Wightman on the meaning of Article 50.
Second, the reasoning and result in Miller does not, in any event, depend upon the assumption which the Supreme Court (and, indeed, Divisional Court) made. That is, in my view, both the reasoning and result in Miller survive even if Article 50 were unilaterally revocable (as the CJEU has today found). Indeed, the Secretary of State in Miller himself accepted this: “it is the Secretary of State’s case that, even if this common ground [that Article 50 is irrevocable] is mistaken, it would make no difference to the outcome of these proceedings” (para. 26 of Miller).
Why is this the case?
The majority judgment in Miller reveals that challenges to the use of purported prerogative powers can take one of four forms (what I have elsewhere called the ‘the four E’s’): first, an examination of whether a prerogative power exists; second, assuming one does exist, the extent of that prerogative power (ie whether it is of a sufficient nature and scope to extend to the relevant circumstances and intended usage envisaged by the executive); third, an examination of whether any such prerogative power has been excluded by a statute or statutory provision (whether expressly or by necessary implication); and fourth, assuming that a sufficient prerogative exists and has not been excluded by primary legislation, an examination of the exercise of that prerogative on grounds of, for example, irrationality, procedural impropriety or disproportionality.
The important point for present purposes is the second category of control concerning a prerogative’s extent. In Miller, after establishing that a relevant prerogative power, the foreign relations treaty prerogative, existed, the Supreme Court moved on to examine the extent of that prerogative power. The Court explored the delimitations of prerogative powers generally, and the foreign relations treaty prerogative specifically, in determining that such executive powers cannot affect domestic law or, separately, domestic rights, and cannot frustrate the purpose of a statutory provision by effectively emptying it of content. The foreign relations treaty prerogative simply does not extend to such circumstances whereby there would be domestic effects. The clue is in the name.
The question therefore became whether the prospective use of a purported prerogative power – here to trigger the Article 50 process – could have any of these three effects, namely: (i) affecting domestic law; (ii) affecting domestic rights; or (iii) frustrating statutory intent. If the answer is “yes”, then the prerogative does not extend to such circumstances and cannot be used due to its inherent nature and scope.
The effect of the Article 50 scheme is that, if nothing else happens, a notifying member state loses that membership – and all entailing benefits – at the end of the two-year period. The Court found that triggering Article 50 has the default effect that domestic law and rights would be affected, if not lost, contrary to Parliament’s intention as expressed in Acts, such as the European Communities Act 1972. This blog is not the place to examine the factual and legal reasons for why the Court found that each of these three effects was made out (see here for further discussion). It is enough for present purposes to note that the default effect – not inevitable effect – of sending the Article 50 Notice would, without anything more, include at least one of the three consequences.
That default, rather than inevitable, effect is sufficient to prohibit the use of the prerogative for two reasons. First, as the foreign relations prerogative cannot by its nature have such effects, the logical possibility – indeed, default consequence – of such attempted usage means that it could not be employed in the first place. The Prime Minister simply did not have the competence to use the prerogative. Second, Parliament cannot be pre-empted or forced to act in order to save its statutes (the highest source of law domestically) on account of executive action. The triggering of Article 50 sets in motion a series of events which, without more, would result – without Parliamentary authorisation – in statutes being made defunct on the Supreme Court’s analysis. Again, the foreign relations treaty prerogative cannot be used to have this effect. With neither a prerogative power nor a pre-existing statutory power enabling it to act, the consequence was that the Government had to seek fresh statutory authorisation from the ‘constitutionally senior partner’, Parliament, before a Minister could notify the European Council of the UK’s intention to leave the EU pursuant to Article 50.
Third, the assumption was prudent as a matter of constitutional adjudication. A question of competence to trigger a process cannot be answered by the courts once it has already been actioned and/or had (irreversible) effects; the logical possibility, known in advance, is enough as a matter of law to prevent the prerogative’s use. In order to answer that competence question, the Court has to ground its analysis in how it is actually envisaged that the prerogative in question is to be used. Here, the Government’s position was (and is) that “as a matter of firm policy, once given a notification will not in fact be withdrawn” (counsel for the Secretary of State during the hearing before the Divisional Court at p.64 of the transcript, 17 October 2016). Accordingly, the Court was being told that if the Government could notify by virtue of a prerogative power, it would not revoke that notification. Effectively, the default effects would become the inevitable effects. That firm policy decision remains despite the outcome in Wightman.
In summary, therefore, Miller remains good law (with consequences beyond only the situation of Brexit, as I explain here ). Neither the result, nor the reasoning, is invalidated in any way. Whilst it obviously assisted the claimants in Miller that the Government conceded the Article 50 revocability question (because it brought to life the effects in domestic law), the core thread survives: the foreign relations treaty prerogative could not have been used to trigger the Article 50 process, regardless of the revocability of any notification, due to its default effects.
Jack Williams was junior counsel for one of the interested parties in R (Miller and Dos Santos) v Secretary of State for Exiting the European Union  UKSC 5 at both Divisional and Supreme court levels. He is co-editor (with Professors Mark Elliott and Alison Young) of the UK Constitution after Miller: Brexit and Beyond (Hart Publishing, 2018).