This paper addresses the question of whether, as a matter of law, Brexit is now unstoppable, without the agreement of the remaining 27 members states of the European Union. In other words, what would happen if, on a date before March 29, 2019, Parliament were to conclude that Britain should not leave the European Union, despite notice of its intention to do so having been given by the prime minister on March 28, 2017? 

There are two parts to this question. The first is whether a formal (and legally binding) decision to leave the European Union has already been taken as a matter of national constitutional law, or whether all that the prime minister has done so far, and all she has had statutory authority to do, is give notice of the present government’s intention to leave. On this matter, this paper’s view is that a further act of Parliament, not just an indicative vote, is needed before a constitutionally valid decision can be taken to leave the European Union. If no such statutory authority is given before March 29, 2019, no constitutionally valid decision to withdraw has been made, and, in any event, the government could withdraw the notification of an intention to leave the European Union and decide to remain.

The second issue is whether, as a matter of EU law, a member state that has given notice of an intention to leave the European Union is bound to leave, or whether it can nonetheless withdraw the notice and decide, unilaterally, to remain. This question requires close consideration of the text of article 50, and what it might mean, and close consideration of the Miller decision. While this paper argues that the better view is that article 50 is unilaterally reversible before the two-year notice period contained in article 50(3) has expired, there is no case law on this question. On this, should it be tested, the Court of Justice of the European Union would be the ultimate arbiter. 

Part of Series

Brexit: The International Legal Implications is a series examining the political, economic, social and legal storm that was unleashed by the United Kingdom’s June 2016 referendum and the government’s response to it. After decades of strengthening European integration and independence, the giving of notice under article 50 of the Treaty on European Union forces the UK government and the European Union to address the complex challenge of unravelling the many threads that bind them, and to chart a new course of separation and autonomy. Brexit necessitates a deep understanding of its international law implications on both sides of the English Channel, in order to chart the stormy seas of negotiating and advancing beyond separation. The paper series features international law practitioners and academics from the United Kingdom, Canada, the United States and Europe, explaining the challenges that need to be addressed in the diverse fields of trade, financial services, insolvency, intellectual property, environment and human rights.
  • Helen Mountfield, Q.C., is a barrister in England, where she is a founder member of Matrix Chambers, which has a strong international presence. She specializes in constitutional and administrative law, and has appeared in many leading cases in the Supreme Court, the Court of Justice of the European Union and the European Court of Human Rights.