Background: Optimal oxytocin dosing during induction of labour remains uncertain despite widespread use. We aimed to determine whether a high-dose oxytocin regimen reduced caesarean section rates without increasing maternal or neonatal morbidity compared with a standard-dose regimen in nulliparous women. Methods: We performed a multicentre, randomised, parallel-group, double-blind controlled trial across 16 UK maternity units, including nulliparous women undergoing induction of labour requiring oxytocin after rupture of membranes. Participants were randomly assigned (1:1) to receive either high-dose oxytocin (starting 4 mU/min, titrated to a maximum dose of 64 mU/min) or standard-dose oxytocin (starting 2 mU/min, titrated to a maximum dose of 32 mU/min). The primary outcome was caesarean section. The trial is registered at the ISRCTN registry, ISRCTN79220656. Findings: Between May 31, 2022, and Sept 7, 2025, 976 women were randomised to high-dose (n=489) or standard-dose oxytocin (n=487). Following a futility analysis requested by the Data Monitoring Committee, recruitment stopped prematurely because of low probability of benefit with high-dose. Caesarean section occurred in 218 (45%) of 489 participants in the high-dose group and 193 (40%) of 487 participants in the standard-dose group (adjusted risk ratio 1· 14, 95% CI 0·99 to 1·32; adjusted risk difference 0·05, 95% CI 0·01 to 0·11; p=0·08). Total dose received was 47% higher in the high-dose group (10·3 mU/min v 7·0 mU/min). Uterine tachysystole was more common with high-dose (208 [43%] versus 167 [35%]; adjusted risk ratio 1·21, 95% CI 1·03 to 1·41; adjusted risk difference 0·10, 95% CI 0·04 to 0·16). Maternal and neonatal outcomes and serious adverse events were similar between groups, with 960 babies discharged home with mother. Interpretation: A high-dose oxytocin regimen, compared to standard-dose for induction of labour did not reduce caesarean section rates. High-dose oxytocin should not be used.