Following the 2016 referendum, the UK notified its intention to withdraw from the European Union pursuant to Article 50 TEU. Given the political and legal consequences of a much-questioned referendum and the strong opinion of many parts of British society that the UK’s membership should not be terminated, the question arose whether such a notification could be revoked unilaterally. In the absence of any mention in Article 50, expert opinion was divided. International law – that is, the law of treaties and the law of international organizations – does not appear to provide a definite answer, while state practice is rather scarce. The constituent instruments of international and regional organizations containing withdrawal clauses are also silent, except for African organizations and development/investment organizations, which invariably allow Member States to rescind withdrawal notices. As regards the EU Treaties, before the Lisbon Treaty they did not contain a withdrawal clause. In the preliminary ruling given in Wightman v. Secretary of State for Exiting the European Union, which concerned whether an EU Member has the sovereign power under Article 50 to revoke unilaterally a withdrawal notice, the Court of Justice helped to clarify a critical question of EU Law but also of international law.