BackgroundAround 30% of post-menopausal breast cancer is related to excess body fat, alcohol intake and low levels of physical activity. Current estimates suggest that there is a 12% increased risk in post-menopausal breast cancer for every 5 kg/m2 increase in body mass index (BMI). Despite this evidence there are few lifestyle programmes directed towards breast cancer risk reduction. This paper describes the process of optimising of the ActWELL programme which aims to support weight management in women invited to attend routine National Health Service (NHS) breast screening clinics.MethodsA feasibility study of a prototype programme aiming to change lifestyle behaviours was successfully undertaken. The programme used educational approaches and behaviour change techniques delivered by lifestyle coaches using individual face to face meetings and telephone sessions. To optimise the intervention for a definitive randomised controlled trial of weight management, data from the feasibility trial, focus group discussions conducted with the target population, feedback from the trial public advisory group and comments from peer reviewers were obtained. Concepts from implementation research provided further guidance to assist in the refinement of the intervention, which was then discussed and agreed by all investigators and the Trial Steering Group.ResultsThe results from the feasibility trial were considered appropriate for moving on to a full trial with 70% of participants finding the programme acceptable. The primary outcomes (weight loss and physical activity) provided an important focus for design input from the target group. The contributions highlighted the need to review programme duration, coach contact time, content and use of behaviour change techniques and communications generally (e.g. science and evidence, non-judgemental approaches and avoiding guilt). In addition, the need for emphasis on support rather than education became apparent. The recommendations from peer reviewers focussed on the magnitude of effort required to achieve the intended weight loss and weight loss maintenance. Implementation science supported the use of the capability/opportunity/motivation (COM-B)model in overall design.ConclusionsThe optimisation process has facilitated the development and evaluation of a programme that enables the delivery of a promising intervention to achieve weight management in post-menopausal women.Trial registrationISRCTN: ISRCTN11057518. Registered on 21 July 2017. Retrospectively registered.