AbstractAimsThe NHS Diabetes Prevention Programme (NHS DPP) is a large‐scale, England‐wide behaviour change programme for people at high risk of progressing to type 2 diabetes. We summarise the findings of our six‐year DIPLOMA evaluation of its implementation and impact and highlight insights for future programmes.MethodsUsing qualitative interviews, document analysis, observation, surveys and large dataset analysis, eight interlinked work packages considered: equity of access; implementation; service delivery and fidelity; programme outcomes; comparative effectiveness and cost‐effectiveness in reducing diabetes incidence; and patient decision making and experience.ResultsDelivery of the NHS DPP encountered barriers across many aspects of the programme, and we identified inequalities in terms of the areas, organisations and patient populations most likely to engage with the programme. There was some loss of fidelity at all stages from commissioning to participant understanding. Despite these challenges, there was evidence of significant reductions in diabetes incidence at individual and population levels. The programme was cost‐effective even within a short time period.ConclusionsDespite the challenge of translating research evidence into routine NHS delivery at scale, our findings suggest that an individual‐level approach to the prevention of type 2 diabetes in a ‘high‐risk’ population was more effective than usual care. By embedding evaluation with programme delivery and working closely with the NHS DPP team, we provided actionable insights for improving communications with potential participants, supporting primary care referral, honing the delivery model with better provider relationships and more patient choice, increasing understanding of behaviour change techniques, and enriching the educational and health coaching content.