UK Supreme Court: Parliament Must Approve Triggering Article 50

In perhaps its most high-profile constitutional law ruling of recent times, the UK Supreme Court has determined that the Government cannot unilaterally trigger Article 50 of the Treaty on the European Union ("Article 50") to commence the UK's withdrawal from the EU (known as "Brexit"). Any decision to do so can only be made by Parliament.

Introduction

On 23 June 2016, the people of the UK narrowly voted to leave the EU by a 52% to 48% majority. In early October 2016, incoming Prime Minister, Teresa May, delivered her pledge to invoke Article 50 by the end of March 2017. Yet, well in advance of that pledge, eagle-eyed constitutional law experts identified a subtle, yet important, question: absent prior Parliamentary authority, is that something the UK Government can actually do?

After a formal legal challenge was lodged in July 2016, the UK Supreme Court has now answered that question: no.

What is Article 50?

Article 50 is the provision of the Treaty of the European Union which sets out the procedure for the withdrawal of a Member State from the EU.

Once invoked, it starts a two-year period during which the EU and the Member State concerned must negotiate and conclude arrangements for the withdrawal. It was common ground between the parties to the litigation that the Article 50 notice, once triggered, is not reversible. The European Council, in agreement with the Member State concerned, can unanimously decide to extend this two-year period.

Crucially, the Article states that a Member State's decision to trigger Article 50 must be made "in accordance with its own constitutional requirements".

So what does that mean for the UK's decision? Does the UK's constitution allow its Government to trigger Article 50 without further ado? Or must that decision first receive Parliament's blessing?

The claimants in the case of R (Miller) v Secretary of State for Exiting the EU [2017] UKSC5 - Gina Miller (a UK investment manager) and Dier Tozetti Dos Santos (a hairdresser) - sought a resolution to precisely this issue.

The legal challenge: First Instance

The case came first before the UK High Court in a three-day hearing in October 2016.

The Government argued that the decision to withdraw the UK from the EU belongs to the realm of foreign affairs - an area of policy that has traditionally been left to the exercise of the executive's "prerogative powers" (a residue of powers exercisable by the Crown (i.e. the Monarch) acting through the executive branch of...