BACKGROUND: COVID-19 lockdown introduced substantial barriers to physical activity, providing a unique 'natural experiment' to understand the social factors associated with sustained physical activity. The objectives of this study were to identify the proportion of people who successfully sustained physical activity during lockdown and to explore whether social support, loneliness and social isolation were associated with maintenance of physical activity during COVID-19 lockdown. METHOD: Longitudinal data from 16,980 participants, mean age 51.3 years (SD = 14.3) from the COVID-19 Social Study was used to identify a sample of participants who maintained their physical activity despite lockdown. RESULTS: Seventeen percent were consistently active whilst 42% were completely inactive. After adjustment for multiple confounders, high social support was associated with a 64% (95% CI 50-80%) increased odds of sustaining physical activity and medium social support was associated with 32% (95% CI 20-44%) increased odds. Associations between physical activity and loneliness and social isolation were not found. CONCLUSION: This study supports previous research showing the importance of social support for the long-term maintenance of physical activity behaviour but shows that such effects extend to contexts of social restrictions.