What exactly Brexit means for British engagement in European external affairs and development cooperation, is highly unclear, while its potential impact is considerable. After the general election in the UK on 8 June 2017, uncertainty regarding the direction, process and timing of the Brexit negotiations and the risk of a disorderly separation have risen further. The government position of a ‘hard Brexit’ seems no longer to be carved in stone. Yet, given the expected – total or partial – withdrawal of a major EU member state, like any area of EU politics, also European development policy faces a number of challenges: short-term problems regarding existing legal obligations, looming budget shortfalls and the securing of business continuity as well as the longer-term realignment of EU development policy following the departure of the United Kingdom. There is also the problem of the UK’s succession in international treaties and mixed agreements in which both the EU and the member states are partners, such as trade agreements and memberships of international organisations, global development financing and representation in multilateral forums or negotiations. By qualitative interpretative analysis, and tracing the leave-process, this article discusses possible scenarios and their consequences of Brexit for UK and EU cooperation with developing countries and their implications within the wider context of EU external action. The analysis builds on new-institutionalism and connects to theories of differentiated integration, framing Brexit as a phenomenon of regional dis-integration and contributing to the development of instruments for studying such phenomena. While, at this point of time, conclusions are speculative in nature, a strategic foresight would advise to protect development policy as far as possible from the trade-offs of the negotiation gamble and place common goals and values beyond dispute.