Objectives To assess the behavioural and weight status outcomes in English children in a feasibility study of a novel primary school-based obesity prevention programme.

Design Exploratory cluster randomised controlled trial of the Healthy Lifestyles Programme.

Setting Four city primary schools (two control and two intervention) in the South West of England.

Participants 202 children aged 9–10 years, of whom 193 and 188 were followed up at 18 and 24 months, respectively. No child was excluded from the study; however, to be eligible, schools were required to have at least one single Year 5 class.

Intervention Four-phase multicomponent programme using a range of school-based activities including lessons, assemblies, parents' evenings, interactive drama workshops and goal setting to engage and support schools, children and their families in healthy lifestyle behaviours. It runs over the spring and summer term of Year 5 and the autumn term of Year 6.

Primary and secondary outcomes Weight status outcomes were body mass index, waist circumference and body fat standard deviation scores (SDS) at 18 and 24 months, and behavioural outcomes were physical activity, television (TV) viewing/screen time and food intake at 18 months.

Results At 18 months of follow-up, intervention children consumed less energy-dense snacks and more healthy snacks; had less ‘negative food markers’, more ‘positive food markers’, lower mean TV/screen time and spent more time doing moderate-vigorous physical activity each day than those in the control schools. Intervention children had lower anthropometric measures at 18 and 24 months than control children, with larger differences at 24 months than at 18 months for nearly all measures.

Conclusions Results from this exploratory trial show consistent positive changes in favour of the intervention across all targeted behaviours, which, in turn, appear to affect weight status and body shape. A definitive trial is now justified.