Objective: To determine the impact of the COVID-19 pandemic on surgery for severe endometriosis in the UK at a national, regional and centre-level. Design: Population-based national cohort study. Population: All women undergoing endometriosis surgery requiring dissection of the pararectal space in the UK from 2017 to 2020 inclusive. Methods: The British Society for Gynaecological Endoscopy (BSGE) collects data nationally on all operations for severe endometriosis which involve dissection of the pararectal space. Annual audits of this database were obtained from the BSGE. Publicly available data on COVID-19 deaths and population were obtained from the UK Office for National Statistics. Main outcome measures: Numbers of annual BSGE-registered endometriosis operations. Results: A total of 5916 operations were performed. The number of operations decreased by 49.4% overall between 2019 and 2020. The number of endometriosis centres remained the same, however the median number of operations per centre decreased from 21 to 12.5, with a median percentage decrease at each centre of 51.0% (IQR 29.4% – 75.0%). There was no significant change in the type of surgery performed. All 11 administrative regions of Great Britain had reduced numbers of BSGE-registered operations in 2020 compared with the average for 2017-2019, with a median 56.6% decrease (range 6.13% - 68.62%). Regional reduction in operations was significantly correlated with COVID-19 death rates (r=0.557, 95% CI of r 0.048 – 1.00, p=0.037). Conclusions: There has been a dramatic fall in the number of operations for severe endometriosis in Britain during the COVID-19 pandemic.