BACKGROUND: Late access to antenatal care is a contributor to excess mortality and morbidity among ethnic minority mothers compared to White British in the UK. While individual ethnicity and socioeconomic disadvantage are linked to late antenatal care initiation, studies have seldom explored patterns of late initiation and associated factors in ethnically dense socially disadvantaged settings. This study investigated disparities in the timing of antenatal care initiation, and associated factors in an ethnically dense socially disadvantaged maternal cohort. METHODS: A retrospective cross-sectional study using routinely collected anonymous data on all births between April 2007-March 2016 in Luton and Dunstable hospital, UK (N = 46,307). Late initiation was defined as first antenatal appointment attended at > 12 weeks of gestation and further classified into moderately late (13-19 weeks) and extremely late initiation (≥ 20 weeks). We applied logistic and multinomial models to examine associations of late initiation with maternal and sociodemographic factors. RESULTS: Overall, one fifth of mothers (20.8%) started antenatal care at > 12 weeks of gestation. Prevalence of late initiation varied across ethnic groups, from 16.3% (White British) to 34.2% (Black African). Late initiation was strongly associated with non-White British ethnicity. Compared to White British mothers, the odds of late initiation and relative risk of extremely late initiation were highest for Black African mothers [adjusted OR = 3.37 (3.05, 3.73) for late initiation and RRR = 4.03 (3.51, 4.64) for extremely late initiation]. The odds did not increase with increasing area deprivation, but the relative risk of moderately late initiation increased in the most deprived ([RRR = 1.53 (1.37, 1.72)] and second most deprived areas [RRR = 1.23 (1.10, 1.38)]. Late initiation was associated with younger mothers and to a lesser extent, older mothers aged > 35 years. Mothers who smoked during pregnancy were at higher odds of late initiation compared to mothers who did not smoke. CONCLUSIONS: There is a need to intensify universal and targeted programmes/services to support mothers in ethnically dense socially disadvantaged areas to start antenatal care on time. Local variations in ethnic diversity and levels of social disadvantage are essential aspects to consider while planning services and programmes to ensure equity in maternity care provision.