question: Background: There is currently major concern about the impact of the global COVID-19 outbreak on mental health. A number of studies suggest that mental health deteriorated in many countries prior to enforced isolation (lockdown), but it remains unknown how mental health has changed during lockdown. Aims: This study explored trajectories of anxiety and depression over the first two months of lockdown using data from the UK, and compared the experiences of individuals with and without diagnosed mental illness. Methods: Data from 53,328 adults in the UCL COVID -19 Social Study (a well-stratified panel study weighted to population proportions collecting data weekly during the Covid-19 pandemic) were analysed from 21/03/2020-10/05/2020. Growth curve modelling was fitted accounting for socio-demographic and health covariates. Results: 24.4% of the sample had scores indicating moderate-severe anxiety, and 31.4% indicating moderate-severe depressive symptoms. Over the first two months of lockdown, there was only a slight decrease in anxiety levels amongst participants as a whole and a very small decrease in depression levels between weeks 3-6 that then increased again in weeks 7-8. Adults with pre-existing diagnoses of mental health conditions had higher levels of anxiety and depression but there was no evidence of widening inequalities in mental health experiences compared to people without existing mental illness. Conclusions: Results suggest there has been little improvement in depression and only slight improvements in anxiety since lockdown commenced in the UK. These findings suggest greater efforts need to be made to help individuals manage their mental health during the pandemic.