When does uncertainty about continuing membership in a highly integrated polity like the EU starts affecting the behaviour of judges and litigants? The Brexit vote in June 2016 has created uncertainty around the application of EU law and the authority of the European Court of Justice in the UK legal system. While the UK is still formally part of the EU, we argue that this uncertainty is affecting the propensity of British judges and litigants, discouraging them to invoke EU law and to refer cases to the European Court of Justice. Using data on referral activity between 1973 and 2017, we apply a difference-in-difference design to evaluate the effect the impact of the Brexit vote on the referral propensity of British courts. We find strong evidence that the Brexit vote is adversely affecting the use of EU law. We conclude that, while politicians are still struggling to define the terms of the UK’s relationship with the EU, the vote has already begun to unravel the fabric of legal integration.