Objective To compare two quality improvement (QI) interventions to improve antenatal magnesium sulphate (MgSO4) uptake in preterm births for the prevention of cerebral palsy. Design Unblinded cluster randomised controlled trial. Setting Academic Health Sciences Network, England, 2018. Sample Maternity units with ≥10 preterm deliveries annually and MgSO4 uptake of ≤70%; 40 (27 NPP, 13 enhanced support) were included (randomisation stratified by MgSO4 uptake). Methods The National PReCePT Programme (NPP) gave maternity units QI materials (clinical guidance, training), regional support, and midwife backfill funding. Enhanced support units received this plus extra backfill funding and unit‐level QI coaching. Main outcome measures MgSO4 uptake was compared using routine data and multivariable linear regression. Net monetary benefit was estimated, based on implementation costs, lifetime quality‐adjusted life‐years and societal costs. The implementation process was assessed through qualitative interviews. Results MgSO4 uptake increased in all units, with no evidence of any difference between groups (0.84 percentage points lower uptake in the enhanced group, 95% CI −5.03 to 3.35). The probability of enhanced support being cost‐effective was <30%. NPP midwives gave more than their funded hours for implementation. Units varied in their support needs. Enhanced support units reported better understanding, engagement and perinatal teamwork. Conclusions PReCePT improved MgSO4 uptake in all maternity units. Enhanced support did not further improve uptake but may improve teamwork, and more accurately represented the time needed for implementation. Targeted enhanced support, sustainability of improvements and the possible indirect benefits of stronger teamwork associated with enhanced support should be explored further.