The UK Supreme Court (UKSC) has today handed down its Judgment in the well-known litigation concerning the UK's constitutional requirements for triggering Art 50 TEU and starting the process of leaving the EU -- see R (on the application of Miller and another) v Secretary of State for Exiting the European Union  UKSC 5 (the Miller Judgment).
The UKSC has ruled that the UK Government cannot trigger Article 50 TEU without previous UK Parliament legislative intervention. In doing so, the UKSC has sought to reassert the basic constitutional principle of Parliamentary sovereignty. However, it has done so in a way that both infringes its duties under EU law and does a disservice to the UK Parliament.
Breach of UKSC's duties under EU law
One of the difficult legal issues on which the Brexit litigation hinged concerned the interpretation of Art 50 TEU and, in particular, the revocability of a notice given under Art 50(2) TEU. The interpretation of this point of law falls within the exclusive competence of the European Court of Justice (ECJ) under Art 263 TFEU. Interestingly, the UKSC stressed this monopoly of interpretation as a key element of EU law at para  of the Miller Judgment: 'so long as the United Kingdom is party to the EU Treaties, UK courts are obliged (i) to interpret EU Treaties, Regulations and Directives in accordance with decisions of the Court of Justice, (ii) to refer unclear points of EU law to the Court of Justice, and (iii) to interpret all domestic legislation, if at all possible, so as to comply with EU law' (emphasis added).
However, the UKSC has violated the ECJ's monopoly of interpretation of the EU Treaties by accepting the parties' commonly agreed position on the irrevocability of an Art 50(2) TEU notice at  of the Miller Judgment:
In these proceedings, it is common ground that notice under article 50(2) (which we shall call “Notice”) ... once given, it cannot be withdrawn. Especially as it is the Secretary of State’s case that, even if this common ground is mistaken, it would make no difference to the outcome of these proceedings, we are content to proceed on the basis that that is correct, without expressing any view of our own on either point. It follows from this that once the United Kingdom gives Notice, it will inevitably cease at a later date to be a member of the European Union and a party to the EU Treaties (emphases added).
In doing so, the UKSC has infringed its obligation under Art 267(3) TFEU to engage in a preliminary reference to the ECJ concerning the interpretation of Art 50 TEU (for legal background see here and here). This cannot be saved by an argument that, under domestic procedural rules (or conventions), the UKSC had the possibility of taking this approach--and effectively dodging one of the most complex and unpredictable legal issues on which the litigation rested.
There are several reasons for this, but the primary one is that, as matter of EU law, a preliminary reference by the highest court of an EU Member State is unavoidable where the interpretation of EU law is necessary to enable it to give judgement--or, in other words, where the judgment relies on a given interpretation of EU law.
In my view, it is beyond doubt that the UKSC Miller Judgment is based on the interpretation that an Art 50(2) TEU notice is irrevocable, and that this represents the legally binding view of the majority judgment, regardless of the attempt to save the UKSC's view on this point in para  -- or, in other words, it is not (logically, legally) true that the UKSC's Miller Judgment operates 'without expressing any view of our own on either point' (ie regarding the revocability or not of the Art 50(2) TEU notice).
There are explicit indications of this interpretation in paras , ,  and , where the Judgment indicates that
... analyse the effect of the 1972 Act and the arguments as to whether, in the absence of prior authority from Parliament in the form of a statute, the giving of Notice by ministers would be ineffective under the United Kingdom’s constitutional requirements, as it would otherwise impermissibly result in a change in domestic law 
... A complete withdrawal represents a change which is different not just in degree but in kind from the abrogation of particular rights, duties or rules derived from EU law. It will constitute as significant a constitutional change as that which occurred when EU law was first incorporated in domestic law by the 1972 Act. And, if Notice is given, this change will occur irrespective of whether Parliament repeals the 1972 Act 
... There is a substantial difference between (i) ministers having a freely exercisable power to do something whose exercise may have to be subsequently explained to Parliament and (ii) ministers having no power to do that thing unless it is first accorded to them by Parliament. The major practical difference between the two categories, in a case such as this where the exercise of the power is irrevocable, is that the exercise of power in the first category pre-empts any Parliamentary action. When the power relates to an action of such importance to the UK constitution as withdrawing from the Treaties, it would clearly be appropriate for the power to be in the second category 
Although its invocation [of Art 50 TEU] will have the inevitable consequence which Lord Pannick described ... [104, all emphases added].
In view of the relevance of the points of irrevocability of the Art 50(2) TEU notice, it is clear to me that the UKSC had an obligation to seek the interpretation of this provision by the ECJ and that, in not doing so, it has breached EU law. Moreover, beyond what some may consider a highly technical or academic point, by not seeking this clarification the UKSC has also done a disservice to the UK Parliament.
Disservice to the UK Parliament
The UK Parliament will imminently enter into debates and legislative action concerning the trigger of the process to withdraw from the EU by serving notice under Art 50(2) TEU. Unless political events unfold in a surprising way, and based on a previous Parliamentary resolution, the UK Parliament should be considered to be politically committed to support the UK Government's stated commitment to trigger Art 50 TEU by the end of March 2017.
However, these debates and eventual Parliamentary decisions will develop under the shadow of uncertainty that remains around the revocability or not of the Article 50(2) TEU. In that regard, the debates and positions expressed by MPs will not be as meaningful as they could if it was clear that the triggering Art 50 TEU would actually put (or not) the UK in the unavoidable course of leaving the EU -- with or without an exit deal, and regardless of the assessment of the fallback position. This can result in the need to make wild assumptions and to necessarily decide on the basis of worse case scenario analysis that may not reflect an alternative (possible) reality of reduced definiteness of the triggering of Art 50 TEU.
The UKSC could have avoided this situation by referring the question for interpretation to the ECJ. On the contrary, by premising its Judgment on the irrevocability of the notice, the UKSC has raised the stakes and the risks of Parliamentary debate even higher and created a situation where decisions are bound to be made in a rushed fashion and in a scenario of all or nothing (perceived) implications of the giving of notice under Art 50(2) TEU. It is hard to see how this can contribute to the practical enablement of Parliamentary sovereignty.