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Five hundred people with severe and complex obesity will be randomly assigned either to receive bariatric surgery or to take part in an intensive weight-loss programme, in a trial running across the UK and Denmark. This matters because bariatric surgery is effective but carries surgical risks, high costs, and limited availability. Meanwhile, intensive lifestyle interventions—combining strict diet, exercise, and behavioural support—have shown promise but lack robust evidence from head-to-head trials against surgery in the most severely obese patients. Clinicians and patients currently have no clear data on whether the non-surgical route can match surgery’s benefits or avoid its harms. If the trial shows that an intensive weight-loss programme achieves comparable weight reduction and health improvements with fewer complications, it could shift clinical guidelines. More patients might avoid major surgery and its recovery burden, while health systems could offer a less invasive, lower-cost alternative. If the lifestyle intervention proves inferior, the results will reinforce the case for prioritising surgical referral. Either way, the evidence will help commissioners and surgeons make better-informed decisions about how to allocate resources for obesity treatment.