This article addresses a critical gap in the literature on winner–loser effects that consists of the lack of attention for highly contentious constitutional referenda. It uses unique multi-wave panel data of over 13,000 people that is unrivalled in size and richness. We estimate causal effects of the referendum on rarely studied but crucial public perceptions of the fairness of the way a referendum is conducted. These perceptions pertain to the highly contentious 2016 European Union (Brexit) referendum in the United Kingdom, which is an ideal-type example of a wider class of referenda for which similar outcomes can be expected. We use difference-in-differences methods and find winner–loser effects of a magnitude far greater than ever observed for general elections. Moreover, we find that these effects not only persist, but even grow over time. The findings have profound implications for the use of such referenda.