A 40-year-old man presents with a 3-month history of intermittent bloody diarrhoea, abdominal cramps, and urgency. He has also experienced weight loss of 4kg. Colonoscopy reveals continuous inflammation extending from the rectum up to the splenic flexure, with loss of vascular pattern, granularity, and contact bleeding. Biopsies confirm chronic inflammatory changes limited to the mucosa. What is the most likely diagnosis?