ABSTRACT Objectives To explore clinically important increases in depression/anxiety from before to during the first UK Covid-19 lockdown and factors related to this change, with a particular focus on ethnic differences. Design Pre-Covid-19 and lockdown surveys nested within two longitudinal Born in Bradford cohort studies. Participants 1,860 mothers with a child aged 0-5 or 9-13, 48% Pakistani heritage Main outcome measures Odds ratios (OR) for a clinically important increase (5 points or more) in depression (PHQ-8) and anxiety (GAD-7) in unadjusted regression analyses, repeated with exposures of interest separated by ethnicity to look for differences in magnitude of associations, and lived experience of mothers captured in open text questions. Results The number of women reporting clinically important depression/anxiety increased from 11% to 20% [10-13%;18-22%] and 10% to 16% [8-11%; 15-18%]) respectively. Increases in depression/anxiety were associated with: loneliness (OR: 8.37, [5.70-12.27]; 8.50, [5.71-12.65] respectively); financial (6.23, [3.96-9.80]; 6.03, [3.82-9.51]); food (3.33 [2.09-5.28]; 3.46 [2.15-5.58]); and housing insecurity (3.29 [2.36-4.58]; 3.0 [2.11-4.25]); a lack of physical activity (3.13 [2.15-4.56]; 2.55 [1.72-3.78]); and a poor partner relationship (3.6 [2.44-5.43]; 5.1 [3.37-7.62]. The magnitude of associations between key exposures and worsening mental health varied between ethnic groups. Responses to open text questions illustrated a complex inter-play of challenges contributing to mental ill health including: acute health anxieties; the mental load of managing multiple responsibilities; loss of social support and coping strategies; pressures of financial and employment insecurity; and being unable to switch off from the pandemic. Conclusions Mental ill health has worsened for many during the Covid-19 lockdown, particularly in those who are lonely and economically insecure. The magnitude of associations between key exposures and worsening mental health varied between ethnic groups. Mental health problems may have longer term consequences for public health and interventions that address the potential causes are needed. STRENGTHS AND LIMITATIONS OF THIS STUDY Three key longitudinal studies have highlighted that the Covid-19 pandemic and lockdowns have had a negative impact on mental health, particular in younger adults, women and those from low socio-economic circumstances, but with participants of predominantly White European ethnicity. The Born in Bradford research programme offers a unique opportunity to investigate the impact of Covid-19 lockdown on mental health in a deprived and ethnically diverse population in whom mental ill health is often reported to be more prevalent. This is a longitudinal study containing linked data collected before the Covid-19 pandemic and during the March-June 2020 lockdown which has allowed us to explore change over that time period in a highly ethnically diverse population, the majority of whom live in the most deprived centiles in the UK. Respondents in this study were mothers of children aged 0-5 and/or 9-13 which may limit the wider generalisability, though our findings are broadly similar (in prevalence and associations) to those from another longitudinal study that included adult men and women. We are not aware of other studies that have explored longitudinal change in mental health from before to during the Covid-19 lockdown in a similar ethnically diverse and deprived population.