Background: Family carers provide vital support for patients towards end-of-life, but caregiving has considerable impact on carers’ own health. The scale of this problem is unknown, as previous research has involved unrepresentative samples or failed to fully capture caregiving close to death. Aim: To quantify level of psychological morbidity and general health among a census sample of carers of people with cancer at end-of-life, compared to population reference data. Design: National 4-month post-bereavement postal census survey of family carers of people who died from cancer, retrospectively measuring carers’ psychological health (General Health Questionnaire-12) and general health (EuroQoL EQ-Visual Analogue Scale) during the patient’s last 3 months of life. Participants: N = 1504 (28.5%) of all 5271 people who registered the death of a relative from cancer in England during 2 weeks in 2015 compared with data from the Health Survey for England 2014 (N = 6477–6790). Results: Psychological morbidity at clinically significant levels (General Health Questionnaire-12 ⩾4) was substantially higher among carers than the general population (83% vs 15%), with prevalence five to seven times higher across all age groups. Overall, carers’ general health scores were lower than population scores, median 75 (interquartile range, 50–80) versus 80 (interquartile range, 70–90), but differences were more marked at younger ages. Female carers had worse psychological morbidity and general health than male carers. Conclusion: Levels of psychological morbidity among family carers during end-of-life caregiving are far higher than indicated by previous research, indicating a substantial public health problem. Consistent assessment and support for carers to prevent breakdown in caregiving may produce cost savings in long term.