Up to one third of patients with Ulcerative Colitis (UC) will require hospitalisation for severe disease (ASUC - Acute Severe Ulcerative Colitis), often within the first 12 months of diagnosis. 30-40% of people admitted to hospital with ASUC will require colectomy during the emergency admission. Investigators will develop a multi-centre prospective inception cohort of patients with ASUC with homogeneously collected detailed longitudinal clinical, endoscopic and laboratory data.