The main objective of this study is to determine whether closed-loop glucose control is superior to standard insulin therapy with continuous glucose monitoring (CGM) in young people (≥16 years) and adults with cystic fibrosis (CF) related diabetes. This is an open-label, multicentre, randomised, single-period, two-arm parallel design study, involving a run-in period followed by a 26 week intervention period during which glucose levels will be controlled either by a hybrid closed-loop system or by participants usual insulin therapy with continuous glucose monitoring. A total of up to 128 young people and adults (aiming for 114 completed participants) with CF related diabetes using insulin will be recruited through outpatient CF and diabetes clinics and other established methods at participating centres. Participants who drop out of the study within the first 4 weeks of the intervention period will be replaced. Participants will receive appropriate training in the safe use of the CGM and closed-loop devices. Participants will have access to the study team during the intervention phase with 24/7 telephone support. The primary outcome is time spent in target range between 3.9 and 10.0 mmol/L as recorded by CGM over the 26 week period. Other key endpoints include time above target glucose range (\>10mmol/L), mean glucose, and HbA1c. Secondary outcomes include time spent with glucose levels below target as recorded by CGM, and other CGM-based metrics in addition to percent of predicted FEV1, body mass index, fasting C-peptide levels, insulin requirements and number of pulmonary exacerbations and hospitalisations. Safety evaluation comprises severe hypoglycaemic episodes, and other adverse and serious adverse events. Psychosocial outcomes include CGM \& closed-loop usage, health-related quality of life questionnaires, burden of diabetes management assessment and semi-structured interviews after participants have had at least three months experience of using the technology. Data will be collected for future health economic analysis.