On 5 December, an eleven justice panel of the Supreme Court of the United Kingdom will hear the appeal against the Divisional Court's landmark Brexit decision (R (on the application of Miller) v Secretary of State for Exiting the European Union [2016] EWHC 2768 (Admin). The Divisional Court held that the Government was not entitled to give notice of its intent to withdraw from the European Union under article 50 of the Treaty on European Union without first putting that matter to Parliament. The appeal is of profound significance with global ramifications.

The Divisional Court began its judgment by emphasising that it was not concerned with the merits or politics of Brexit. Appropriately, the Divisional Court relied on The Case of Proclamations (1610) 12 Co Rep 74, in which Sir Edward Coke defied the power of King James I in holding that the King "had no prerogative, but that which the law…allows him". Coke's decision was seen as inherently political - so too the Divisional Court's, which has attracted widespread and unjustified criticism with no small degree of historical continuity.

The principle from The Case of Proclamations was determinative in the Divisional Court. The Court held that the European Communities Act 1972 (ECA), which gives domestic effect to European Community law in the national legal systems of the United Kingdom, was of such constitutional significance, and so entangled with domestic law, that Parliament had displaced the usual powers of the Crown to conduct international relations (ie trigger article 50) without recourse to Parliament.

The Crown's written argument on the appeal relies on the 'dualist' nature of Britain's legal system. It argues that the Crown is free to conduct international affairs under royal prerogative, and that the prerogative can alter domestic law. Under this view, the ECA is merely a conduit of international law, which enables the Crown to trigger article 50 as it pleases. The respondents' written arguments take the position that by the ECA, Parliament implemented major constitutional changes and statutory rights that only Parliament may undo. As a result, the Crown cannot trigger article 50 without Parliament's consent. The Scottish Government's written argument takes the point that if the Crown were to trigger article 50, it would be interfering in a 'devolved matter' for which it would first require the Scottish Parliament's consent. Overall, these arguments turn on how one sees European law - is it a fundamental part of the United Kingdom's constitutional fabric? Or is it merely foreign law of which the Crown may wash its hands? Case law would appear to support the former proposition.

There are, still, further difficulties. Article 50 is part of European law, and therefore justiciable by the European Court of Justice - which, in an ironic twist, may end up having the final say on Brexit. If Parliament's consent is required - there are also questions as to what consent is effective. Lady Hale has opined, extra-judicially, that the constitutional nature of the ECA may require some special form of repeal.

For now, the Government has drafted a 'bombproof' three-line bill to obtain Parliament's approval in the event the appeal should fail. Assuming that the bill is sufficient - the Government must still convince the House of Lords, where peers are said to be strongly opposed to Brexit.

See the Court's decision here.