Objective Investigate the impact of the COVID-19 lockdown on feelings of loneliness and social isolation in parents of school-age children Design Cross sectional online survey of parents of primary and secondary school-age children Setting Community setting Participants 1214 parents of school-age children in the United Kingdom Methods An online survey explored the impact of lockdown on the mental health of parents with school-age children, and in particular about feelings of social isolation and loneliness. Associations between the UCLA three-item Loneliness Scale (UCLATILS), the Direct Measure of Loneliness (DMOL) and the characteristics of the study participants were assessed using ordinal logistic regression models Main outcome measures Self-reported measures of social isolation and loneliness using UCLATILS and DMOL. Results Half of respondents felt they lacked companionship, 45% had feelings of being left out, 58% felt isolated and 46% felt lonely. The factors that were associated with higher levels of loneliness on UCLATILS were female gender, parenting a child with special needs, lack of a dedicated space for distance learning, disruption of sleep patterns and low levels of physical activity during the lockdown. Factors associated with a higher DMOL were female gender, single-parenting, parenting a child with special needs, unemployment, low physical activity, lack of a dedicated study-space and disruption of sleep patterns during the lockdown. Conclusions The COVID-19 lockdown has increased feelings of social isolation and loneliness among parents of school-age children. Two modifiable health seeking lifestyle behaviours (increased levels of physical activity and the maintenance of good sleep hygiene practices) were identified as key factors in reducing feelings of social isolation and loneliness during lockdown.