Objectives: Established in March 2020, the C19PRC Study monitors the psychological and socio-economic impact of the pandemic in the UK and other countries. This paper describes the protocol for Wave 6 (August-September 2021). Methods: The survey assessed: COVID-19 related experiences; experiences of common mental health disorders; psychological characteristics; and social and political attitudes. Adult participants from any previous wave (N = 3170) were re-invited, and sample replenishment procedures helped manage attrition. Weights were calculated using a survey raking algorithm to ensure the on-going original panel (from baseline) was nationally representative in terms of gender, age, and household income, amongst other factors. Results: 1643 adults were re-interviewed at Wave 6 (51.8% retention rate). Non-participation was higher younger adults, those born outside UK, and adults living in cities. Of the adults recruited at baseline, 54.3% (N = 1100) participated in Wave 6. New respondent (N = 415) entered the panel at this wave, resulting in cross-sectional sample for Wave 6 of 2058 adults. The raking procedure re-balanced the longitudinal panel to within 1.3% of population estimates for selected socio-demographic characteristics. Conclusions: This paper outlines the growing strength of the publicly available C19PRC Study data for COVID-19-related interdisciplinary research.