Our forecast, published in the UK chapter of this Review, is conditioned on the assumption that the result of the 23 June referendum is a vote to remain in the EU. The discussion of the economic impact in the first half of the year, and the accompanying uncertainty due to the very act of having the vote, is discussed in the UK chapter in this Review.However, there exists a significant possibility of a vote to leave the EU. The future is, by definition, uncertain and we normally represent this with a distribution of potential outcomes around our modal path for the economy. The referendum presents a particular instance where the future may be genuinely considered bi-modal, with two distinct paths. The outcome of the referendum will determine which of these future paths the UK economy takes.This note presents a simulation exercise designed to give a counterfactual of a world in which the UK votes to leave the EU. We discuss the short-run developments that are most likely to affect the UK economy in the immediate aftermath of a leave vote. We do this by introducing a range of shocks to our global econometric model designed to capture the effects of the UK leaving the EU. These shocks are layered together with a series of more long-run structural changes which are discussed by Ebell and Warren, in this Review.Focusing on the near-term implications, our analysis suggests that the level of GDP in 2017 will be 1 per cent lower than our baseline forecast presented in the UK section of this Review. By 2018 this loss of output widens to 2.3 per cent. Heightened risk and uncertainty will cause sterling to depreciate by around 20 per cent immediately following the referendum, which will result in an intense bout of inflationary pressure. Meanwhile, the same uncertainty induces a tightening of credit conditions and a fall in domestic demand as consumption and investment fall relative to the counterfactual of a vote to remain.We begin by detailing the process by which the UK would negotiate exiting the EU followed by a comprehensive exposition of the shocks that form the core of our shortrun analysis. We then conclude with the quantitative implications of our simulation exercise, macroeconomic policy responses and a discussion of the transition to the longer run, which is discussed in detail in Ebell and Warren, in this Review.