Introduction: Emerging evidence suggests that the COVID-19 pandemic and associated lockdown restrictions may have influenced alcohol consumption. This study examines changes in high-risk alcohol consumption from before to during the COVID-19 crisis in an established cohort of middle-aged British adults. Methods: Participants consisted of 3,358 middle-aged adults from the 1970 British Cohort Study who completed the Alcohol Use Disorders Identification Test for detecting hazardous drinkers in primary care settings in 2016−2018 (when aged 46−48 years) and May 2020 (aged 50 years). Multivariable logistic regression analysis was used to examine changes in high-risk drinking (scores of ≥5), and multinomial regression was used to compare responses with individual test items in 2016−2018 and May 2020. Results: Among middle-aged British adults, high-risk drinking increased by 5.2 percentage points from 19.4% to 24.6% (p<0.001) between 2016−2018 and May 2020. The increase in high-risk drinking was not moderated by sex, marital status, educational attainment, the presence of a chronic illness, or the year the baseline survey was completed. The prevalence of drinking ≥4 times a week doubled from 12.5% to 26% from before to during the pandemic (p<0.001), and there was also evidence of an increase in the frequency of being unable to stop drinking. Conclusions: This study provides evidence linking the COVID-19 crisis and associated lockdown restrictions to an increase in high-risk drinking patterns and particularly frequent drinking in British adults. Potential long-term changes in drinking habits should be monitored following the emergence of the COVID-19 pandemic.