The UK House of Lords has increasingly attracted public attention in recent years because of several government defeats of bills related to the UK’s departure from the European Union (EU). Despite this development, there is little research on how vote coalitions in the UK’s second chamber have transformed since its reform in 1999. In this paper, we address this gap by shedding light on the transformation of Lords’ voting behavior between 2000 and 2020. We argue that the growing bipolarity of the British party system along the UK’s future relationship with the EU had substantive repercussions for decision making processes in the House of Lords. Analysing over 2,000 divisions with optimal classification roll call models (OC), we show that, in the post-Brexit period, partisan clusters among Lords have dissolved, new coalitions in the House of Lords emerged, and Lords’ ideological space became significantly more unidimensional. Our results suggest that the House of Lords has undergone a process of ideological realignment after the 2016 Brexit referendum.