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.
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  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 Government). Accordingly, it said that it could trigger Article 50 through the use of those powers, without prior approval from Parliament.
Successfully challenging this position, the claimants argued that bypassing Parliament in the way the Government intended runs contrary to the UK's constitutional requirements, since it would frustrate "the rights and duties enacted by Parliament" in the European Communities Act 1972 (the "ECA") (the domestic legislation by which the UK joined the EU in 1973). Moreover, they said, it would be "inconsistent with the object and purpose of that Act, namely to give effect to the rights and duties consequent on membership of the EU."
In handing down its decision, the Court held that the Government "does not have the power under the Crown prerogative to give notice pursuant to Article 50..."
The legal challenge: The Supreme Court
Such was the significance of this decision, the Government was granted leave to appeal directly to the Supreme Court (the highest court in the UK), leapfrogging the Court of Appeal in the process. For the first time in the Supreme Court's history, all 11 of its permanent Justices presided over the appeal hearing from 5 to 8 December 2016.
After considering similar arguments on both sides, the Supreme Court handed down its decision on 24 January 2017, dismissing the Government's appeal by a majority of 8 to 3.
The Supreme Court found that, through the enactment of the ECA, EU law had become a source of UK law which takes precedence over all other domestic sources of UK law. Withdrawing would fundamentally change the UK's constitutional arrangements by removing the source of EU law and the UK's constitution requires that such changes can only take effect through primary legislation.
Likewise, the Supreme Court found that withdrawal would also remove existing domestic rights enjoyed by UK residents. While conduct over foreign affairs may well be reserved to executive Government, its prerogative powers do not extend to matters of domestic law absent prior Parliamentary authority.
Accordingly, the Supreme Court has resolved the UK's recent constitutional controversy: an Act of Parliament is first required to authorise the triggering of Article 50.
What next for Brexit?
As the Supreme Court was at pains to emphasise, their ruling has nothing to do with the "wisdom of the decision" to withdraw from the EU or the terms of that withdrawal – which are political issues for ministers and Parliament to resolve.
Indeed, with the legal position now settled, the matter now shifts back to the political sphere. Draft legislation authorising Teresa May's Government is being debated before both houses of UK Parliament during February 2017 – an initial bill (containing just 137 words) was published on 26 January 2017 with an expedited timetable. The view of most political commentators seems to be that a vote in favour will ultimately carry (albeit, potentially with material amendments as the various political stakeholders press to impose their conditions). Indeed, members of the first house passed a preliminary vote on 1 February 2017. There is also the prospect of a possible further vote on the terms of withdrawal at the end of the negotiations (something promised by Ms May shortly before the recent ruling).
While the overall process still remains very much in its infancy, those wishing to track how the matter develops can find up-to-date information and commentary at the Herbert Smith Freehills Brexit Hub.
For now, for Japanese companies with an interest in the timing of Brexit, the message remains: watch this space.
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