This article argues that women welfare claimants, particularly lone mothers, were disproportionately targeted by invasive and punitive anti-fraud measures in the post-1945 welfare state because they were considered most likely to commit fraud through their perceived sexual immorality and reckless childbearing. Although the issue of welfare fraud has influenced high-profile policy and legislation, existing historical scholarship is scant and we know little about the form, extent, or consequences of welfare fraud. Here, a mix of correspondence, statistics, and reports produced by the National Assistance Board, Ministry of Pensions and National Insurance, and National Assistance Board for Northern Ireland are analyzed alongside memoirs and a sample of offenses dealt with by the police to explore the nature and scale of welfare fraud by women alongside an examination of anti-fraud measures employed by the state in Britain and Northern Ireland. It shows that fraud policies risked bringing women into unwelcome conflict with authorities and pushed women into dropping their claims, out of fear of prosecution. In Britain, this approach became intertwined with an embedded hostility toward migrants, while sectarianism underpinned the delivery of state welfare in Northern Ireland. Consequently, these findings suggest new ways of understanding how the welfare state reshaped the relationship between central government and its most vulnerable citizens.