Cancellation of planned surgery impacts substantially on patients and health systems. This study describes the incidence and reasons for cancellation of inpatient surgery in the UK NHS.We conducted a prospective observational cohort study over 7 consecutive days in March 2017 in 245 NHS hospitals. Occurrences and reasons for previous surgical cancellations were recorded. Using multilevel logistic regression, we identified patient- and hospital-level factors associated with cancellation due to inadequate bed capacity.We analysed data from 14 936 patients undergoing planned surgery. A total of 1499 patients (10.0%) reported previous cancellation for the same procedure; contemporaneous hospital census data indicated that 13.9% patients attending inpatient operations were cancelled on the day of surgery. Non-clinical reasons, predominantly inadequate bed capacity, accounted for a large proportion of previous cancellations. Independent risk factors for cancellation due to inadequate bed capacity included requirement for postoperative critical care [odds ratio (OR)=2.92; 95% confidence interval (CI), 2.12-4.02; P<0.001] and the presence of an emergency department in the treating hospital (OR=4.18; 95% CI, 2.22-7.89; P<0.001). Patients undergoing cancer surgery (OR=0.32; 95% CI, 0.22-0.46; P<0.001), obstetric procedures (OR=0.17; 95% CI, 0.08-0.32; P<0.001), and expedited surgery (OR=0.39; 95% CI, 0.27-0.56; P<0.001) were less likely to be cancelled.A significant proportion of patients presenting for surgery have experienced a previous cancellation for the same procedure. Cancer surgery is relatively protected, but bed capacity, including postoperative critical care requirements, are significant risk factors for previous cancellations.