Critical illness-related cardiac arrest (CIRCA) as a distinct entity is not well described epidemiologically. There is currently a knowledge gap regarding how many occur in the UK or the impact on patient outcome. The CIRCA study is a prospective multi-centre observational cohort study of patients in the United Kingdom experiencing a cardiac arrest while in a Critical Care Unit embedded in the Case Mix Programme and National Cardiac Arrest Audit. The duration of data collection is 12 months, with surviving patients and family members receiving questionnaire follow-up at 90 days, 180 days and 12 months. This paper describes the protocol for the CIRCA study which received favourable ethical opinion from South Central – Berkshire Research Ethics Committee and approval from the Health Research Authority. Study registration is on clinicaltrials.gov (NCT04219384).