Leaving the European Union raises the most challenging questions of constitutional law in modern British legal history. The first of them is whether parliamentary approval is needed before the United Kingdom can give notification of an intention to leave. The answer is that an act of parliament is required.

Article 50 of the EU Treaty on European Union says that “any member state may decide to withdraw from the union in accordance with its own constitutional requirements”. Once Article 50 is invoked by the withdrawing state, there is a maximum of two years for the “arrangements for withdrawal” to be negotiated (unless the UK and all the countries of the EU unanimously agree on an extension of time). Article 50 adds that the EU treaties