The United Kingdom faces demographic uncertainty, as negotiations for leaving the European Union (Brexit) proceed. Brexit has implications for international migration into and out of the UK, dependent on future immigration policy and on how attractive the UK will be as a labour market. At the same time, the UK population is experiencing ethnic diversification, consequent on past immigration. To explore the UK's future ethnic diversity, we run four projection scenarios. Three international migration scenarios, varying by the extent of the break with the EU, are implemented together with a reference projection assuming zero international migration. Ethnic groups are differently affected by these migration scenarios, depending on the contribution of international migration to population growth and the extent of demographic momentum. The White British and Irish lose population under all Brexit scenarios and the Black Caribbean population declines in all but one scenario. The White Other, Indian, Chinese, Other Asian and Other groups will show much lower growth under Soft and Hard Brexit scenarios. The growth of the Mixed, Pakistani, Bangladeshi, Black African and Black Other groups will only be affected marginally. Under every scenario, however, the UK's population is projected to continue to grow, age and diversify.