Since 1 January 2021, the ‘one-stop shop’ principle under the EU Merger Regulation (EUMR) no longer applies to the UK, and UK turnover is no longer relevant for determining whether a merger satisfies the EUMR jurisdictional thresholds. Merger control analysis will now need to factor in possible interactions with both the European Commission (Commission) and the Competition and Markets Authority (CMA). The two regimes have different procedures, timelines and substantive tests, which will impact on deal planning and strategy. Dual reviews by both authorities will likely lead to an additional burden on merging parties as well as a risk of a deal being cleared by the Commission but blocked by the CMA, or vice-versa. This article assesses the procedural and practical implications of these changes and highlights some of the key risks businesses may face in merger control looking ahead.