The Court of Justice of the European Union (CJEU) has published today a new set of Recommendations to national courts and tribunals, in relation to the initiation of preliminary ruling proceedings  OJ C 439/1. This is a very timely document, which clarifies the circumstances under which a reference for a preliminary ruling can or must take place, as well as the basic parameters that those requests must meet in order to effectively enable the CJEU to issue preliminary rulings, including in cases requiring particularly expeditious handling.
In the context of the Brexit related litigation before the UK Supreme Court in the appeal of the High Court's Miller decision, these Recommendations are particularly timely and relevant. There has been a very intense discussion by distinguished legal scholars about the existence or not of an obligation to refer the case to the ECJ for interpretation of Article 50 TEU--and, in particular, in relation with the (ir)revocability of an Art 50(1) notice in view of Art 50(2) TEU. The positions are too wide to discuss here (see this very useful compilation of materials), and I hold the relatively minoritarian view that the UK Supreme Court is under an absolute and inexcusable obligation to request a preliminary ruling on the interpretation of Article 50 TEU.
I find additional support for my view and the underlying interpretation of the CILFIT test in para  of the CJEU's Recommendations, which very clearly indicates that:
Where a question is raised in the context of a case that is pending before a court or tribunal against whose decisions there is no judicial remedy under national law, that court or tribunal is nonetheless required to bring a request for a preliminary ruling before the Court (see third paragraph of Article 267 TFEU), unless there is already well-established case-law on the point or unless the correct interpretation of the rule of law in question admits of no reasonable doubt (emphasis added).
Para  is also relevant in its stress that:
The jurisdiction of the Court to give a preliminary ruling on the interpretation or validity of EU law is exercised exclusively on the initiative of the national courts and tribunals, whether or not the parties to the main proceedings have expressed the wish that a question be referred to the Court (emphasis added).
In my opinion, the CJEU has sent the clearest possible message to the UK Supreme Court: they expect a request for an interpretation of Article 50 TEU. And the UK Supreme Court will be well advised to do so as soon as possible, once all intervening parties have presented their arguments. Tertium non datur.