Classroom-based physical activity(PA)interventions offer the opportunity toincrease PA without disrupting the curriculum. We aimedto explore the feasibility and potential effectivenessof a classroom-based intervention on moderate to vigorous PA(MVPA)and total PA. The secondary aim was to assess the acceptability and sustainability of the intervention.In a mixed-methods, non-randomised,exploratory controlled before-and-after study, 152children (10 ± 0.7 years) were recruited from fiveschools; two intervention (n=72) and three control (n=80) schools. Schoolteachers delivered an 8-week classroom-basedintervention, comprising of 10 minutes daily MVPA integrated into the curriculum. The control schools maintained their usual school routine. Mean daily MVPA(min), total PA(mean cpm), physical fitness,and health-related quality of life measurements were taken at baseline, end of intervention,and 4-weekspost-intervention(follow-up). Data were analysed using a constrained baseline longitudinal analysis model accounting for the hierarchical data structure. For the primary outcomes (MVPAand total PA) the posterior mean differenceand 95% compatibility interval were derived using a semi-Bayesian approach with an explicit prior.The acceptability and sustainability of the intervention was explored via thematic content analysis of focus group discussions with teachers (n=5) and children (n=50).The difference in mean daily MVPA (intervention-control) was 2.8(-12.5to 18.0)min/dayat 8 weeks and 7.0 (-8.8 to 22.8) min/day atfollow-up.For total PA, the differences were -2 (-127 to 124) cpm at 8-weeks and 11 (-121 to 143) cpm at follow-up. The interval estimates indicate that meaningful mean effects (both positive and negative) as well as trivial effects are reasonably compatible with the data and design. The intervention was received positivelywithcontinuation reported by the teachers and children. Classroom-based PAcould hold promise for increasingaverage dailyMVPA, but a large cluster randomised controlled trial is required.