Evidence regarding the impact of COVID-19 on health behaviours is limited. In this prospective study including 1.1 million UK and US participants we collected diet and lifestyle data ‘pre-’ and ‘peri-’ pandemic, and computed a bi-directional health behaviour disruption index. We show that disruption was higher in the younger, female and socioeconomically deprived (p<0.001). A loss in body weight (-0.57kg) was greater in highly disrupted individuals compared to those with low disruption (0.01kg). There were large inter-individual changes observed in all 46 health and diet behaviors measured peri-pandemic versus pre-pandemic, but no mean change in the total population. Individuals most adherent to unhealthy pre-pandemic health behaviours improved their diet quality (0.93units) and weight (-0.79kg) compared with those reporting healthy pre-pandemic behaviours (0.08units and 0.04kg respectively), irrespective of relative deprivation. For a proportion of the population, the pandemic may have provided an impetus to improve health behaviours.