Objective: To characterize the clinical features of patients with severe COVID-19 in the UK. Design: Prospective observational cohort study with rapid data gathering and near real-time analysis, using a pre-approved questionnaire adopted by the WHO. Setting: 166 UK hospitals between 6th February and 18th April 2020. Participants: 16,749 people with COVID-19. Interventions: No interventions were performed, but with consent samples were taken for research purposes. Many participants were co-enrolled in other interventional studies and clinical trials. Results: The median age TRANS was 72 years [IQR 57, 82; range 0, 104], the median duration of symptoms before admission was 4 days [IQR 1,8] and the median duration of hospital stay was 7 days [IQR 4,12]. The commonest comorbidities were chronic cardiac disease MESHD (29%), uncomplicated diabetes (19%), non-asthmatic chronic pulmonary disease MESHD (19%) and asthma MESHD asthma HP (14%); 47% had no documented reported comorbidity. Increased age TRANS and comorbidities including obesity MESHD obesity HP were associated with a higher probability of mortality. Distinct clusters of symptoms were found: 1. respiratory ( cough MESHD cough HP, sputum, sore throat, runny nose, ear pain HP pain MESHD, wheeze HP, and chest pain MESHD chest pain HP); 2. systemic ( myalgia MESHD myalgia HP, joint pain MESHD pain HP and fatigue MESHD fatigue HP); 3. enteric ( abdominal pain MESHD abdominal pain HP, vomiting MESHD vomiting HP and diarrhoea). Overall, 49% of patients were discharged alive, 33% have died and 17% continued to receive care at date of reporting. 17% required admission to High Dependency or Intensive Care Units; of these, 31% were discharged alive, 45% died and 24% continued to receive care at the reporting date. Of those receiving mechanical ventilation, 20% were discharged alive, 53% died and 27% remained in hospital. Conclusions: We present the largest detailed description of COVID-19 in Europe, demonstrating the importance of pandemic preparedness and the need to maintain readiness to launch research studies in response to outbreaks. Trial documentation: Available at https://isaric4c.net/protocols . Ethical approval in England and Wales (13/SC/0149), and Scotland (20/SS/0028). ISRCTN (pending).