Many believe the UK to have been an awkward member of the EU, firmly attached to a traditional idea of parliamentary sovereignty. In fact, the UK’s membership of the EU has coincided with a profound transformation in the British state and corresponding understandings of sovereignty. This article examines this transformation by focusing on its legal and constitutional aspects. The article contends that EU membership has provided a context for ‘constitutional modernization’ in the UK, undertaken in the absence of public scrutiny or debate. By the EU referendum of June 2016, the UK found itself with a transformed constitutional structure in practice but with an unchanged reliance in theory on a traditional notion of parliamentary sovereignty. Events over the last 3 years have revealed the depth of this disjuncture and its implications for British sovereignty.