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Objective: To investigate labour duration in different phases of labour when adhering to Zhang's guideline for labour progression compared with the WHO partograph.Design: A secondary analysis of a cluster randomised controlled trial.Setting: Fourteen Norwegian birth care units, each with more than 500 deliveries per year constituted the clusters.Participants: A total of 7277 nulliparous women with singleton foetus in a cephalic presentation and spontaneous onset of labour at term were included.Intervention: Seven clusters were randomised to the intervention group that adhered to Zhang's guideline ( n = 3972) and seven to the control group that adhered to the WHO partograph ( n = 3305) for labour progression. Measurements:The duration of labour from the first registration of cervical dilatation ( ≥ 4 cm) to the delivery of the baby and the duration of the first and second stages of labour; the time-to-event analysis was used to compare the duration of labour between the two groups after adjusting for baseline covariates.Findings: The adjusted median duration of labour was 7.0 h in the Zhang group, compared with 6.2 h in the WHO group; the median difference was 0.84 h with 95% confidence interval [CI] (0.2-1.5). The adjusted median duration of the first stage was 5.6 h in the Zhang group compared with 4.9 h in the WHO group; the median difference was 0.66 h with 95% CI (0.1-1.2). The corresponding adjusted median duration of the second stage was 88 and 77 min; the median difference was 0.18 h with 95% CI (0.1-0.3).