Background: Food supply concerns have featured prominently in the UK response to the COVID-19 pandemic. We assess changes in food insecurity in the UK population from April to July 2020. Method: We analyze 11,095 respondents from the April through July waves of the Understanding Society COVID-19 longitudinal study survey linked with Wave 9 of the UK Understanding Society study. Food insecurity was defined as having used a food bank in the last 4 weeks; being hungry but not eating in the last week; or not able to eat healthy and nutritious food in the last week. Unadjusted estimates to examine changes in population prevalence and logistic regression were used to assess the association between employment transitions and food insecurity. Findings: The prevalence of reporting at least one form of food insecurity rose from 7.1% in April to 20.2% by July 2020. Some of the largest increases were among Asian respondents (22.91 percentage points), the self-employed (15.90 percentage points), and 35-44-year-olds (17.08 percentage points). In logistic regression models, those moving from employment to unemployment had higher odds of reporting food insecurity relative to furloughed individuals (OR=2.23; 95% CI: 1.20 to 4.13) and to the persistently employed (OR=2.38; 95% CI: 1.33 to 4.27), adjusting for sociodemographic characteristics. Furloughed individuals did not differ significantly in their probability of experiencing food insecurity compared to the persistently employed (OR=1.07; 95% CI: 0.83 to 1.37). Interpretation: Food insecurity has increased substantially in the UK. Steps are needed to provide subsidies or food support to vulnerable groups.