Background This study aimed to understand the extent of household food insecurity amongst undocumented migrant families in Birmingham,UK. Methods Cross-sectional survey of households (n = 74) with dependent children using the USDA 18-item household food security (HFS) module. All households had an irregular immigration status and were accessing an immigration advice drop-in service (n = 98 adults; n = 138 children) in Birmingham. Results About 95.9% of households were food insecure, and 94.6% of children lived in households with low or very low food security. Food insecurity varied within households. Around 91.8% of adults were food insecure, compared to 75.6% of children. Spearman's rank-order correlation indicated a statistically significant positive correlation between household food insecurity level and number of children (rho = 0.253, P = 0.031). A Kruskal–Wallis H Test indicated no statistically significant difference (P = 0.730) in HFS score between households supported by asylum support, children's social services or paid employment in the informal economy and those that had no regular income. Conclusions Prevalence of HFS was higher in this sample of undocumented migrant households with dependent children in Birmingham, UK, than in the wider population, and larger households were more food insecure. Households without a regular income were no more likely to be food insecure than households with financial support. [ABSTRACT FROM AUTHOR]