AbstractThis study provides a detailed picture of how the on‐going challenge of teacher shortages in England and Wales is driving the deployment of teaching assistants (TAs) to cover classes in place of teachers. Analyses of data from a survey of nearly 6000 TAs in mainstream and special schools found that TAs cover classes for up to 4 h a week, with one in four covering classes because schools do not have enough teachers and/or are unable to get external supply teachers. The conditions under which TAs cover classes are challenging and a cause of stress and anxiety. Unlike teachers, TAs cover classes without a lesson plan or support from another TA. The majority of TAs said that covering classes inevitably involves them having to teach pupils – despite national guidance stipulating that cover by TAs should not involve ‘active teaching’. Three in four TAs report that their own role is not covered when they cover for teachers. Being deployed to cover classes disrupts and diverts TAs from carrying out their regular duties of delivering curriculum interventions and providing classroom support. Consequently, TAs feel that undertaking cover negatively impacts the quality of learning and provision for pupils with additional needs, as well as their workload, wellbeing, sense of effectiveness and job satisfaction. The conclusion that TAs actively teach lessons to whole classes has potentially significant real‐world consequences and implications for policy and practice. This paper calls for an urgent policy response to what is, in essence, a troubling symptom of the current teacher recruitment and retention crisis.