BACKGROUND: The prevalence of overweight and obesity in young children rose sharply during the COVID-19 pandemic. Here we estimate the potential future health and economic effects of these trends in England. METHODS: Using publicly available annual Body Mass Index (BMI) data from 2006-2022, we calculated the increase in overweight/obesity prevalence (BMI ≥85th reference percentile) during the COVID-19 pandemic among children aged 4-5 and 10-11, and variation by deprivation and ethnicity. We projected the impact of child BMI trends on adult health measures to estimate added lifelong medical and social costs. RESULTS: During 2020-2021 there were steep increases in overweight and obesity prevalence in children. By 2022, overweight and obesity prevalence in children aged 4-5 returned to expected levels based on pre-pandemic trends. However, overweight and obesity prevalence in children aged 10-11 persisted and was 4 percentage points (p<0.001) higher than expected, representing almost 56,000 additional children. The increase was twice as high in the most compared with the least deprived areas. The additional lifelong healthcare cost in this cohort will amount to £800 million with a cost to society of £8.7 billion. We did not find an increase in maternal obesity associated with the COVID-19 pandemic, however, prevalence grew faster in the post pandemic period. DISCUSSION: The return of overweight and obesity prevalence to pre-pandemic trends in children aged 4-5 provides a clear policy target for effective intervention to tackle this growing and serious population health concern.