ABSTRACT In the context of the debate surrounding and following the Brexit referendum in June 2016, credentialed economic expertise did not enjoy a privileged position within the political and policy debate. Using the contrasting cases of the Office for Budget Responsibility and Economists for Brexit/Free Trade, this paper explores the mobilisation of economic expertise within the discursive politics of Brexit under conditions of pervasive radical uncertainty. It argues that politicisation is an important factor in side-lining technocratic influence over policy choice, but that the particular type of politicisation in play (plebiscitary) meant that the input of technocratic experts was downgraded. It is a politics conducted in another register to the reasoned, evidence-based vernacular of accredited experts and the policy discourses that they work with and through. We also argue that the basic unknowability of the post-Brexit economy further impaired expert input. These limitations were acknowledged by accredited experts themselves, reducing their traction within the policy debate. This unknowability was exploited by forms of counter-expertise mobilised by the Leave campaign. The radical uncertainties of Brexit allowed in heterodox assumptions and approaches to compete on a level playing field with an approximation of what could be presented as prudent best practice.