Media organizations can shape news via their choice of coverage and slant. We measure the aggregate slant of the BBC from 2014-18, benchmark it against UK newspapers of known political leaning, and show that it took a sharp turn to the right in the runup to the Brexit vote. We disaggregate this shift across topics and show that roughly three-quarters of the measured change was due to stories related to the European Union, that 90% of that change was due to increased coverage relative to topic-specific slant, and that the change was driven by crowding out of left-leaning topics. We show even stronger results on a Leave/Remain rather than Labour/Conservative scale. We measure the causal effect of this shift using two different datasets and empirical approaches and both show causal increases of exposure to BBC news on Brexit vote intentions/vote shares, with effects sufficiently large that the absence of a change in the BBC’s slant would have made the Brexit vote too close to call.