Objectives The COVID-19 Psychological Research Consortium (C19PRC) Study was established in March 2020 to monitor the psychological and socio-economic impact of the pandemic in the UK and other countries. This paper describes the protocol for Wave 5 (March–April 2021). Methods The survey assessed: COVID-19 related experiences; experiences of common mental health disorders; psychological characteristics; and social and political attitudes. Adults who participated in any previous wave (N = 4949) were re-invited to participate. Weights were calculated using a survey raking algorithm to ensure the longitudinal panel was nationally representative in terms of gender, age, and household income, amongst other factors. Results Overall, 2520 adults participated. A total of 2377 adults who participated in the previous survey wave (November–December 2020) were re-interviewed at Wave 5 (61.5% retention rate). Attrition between these two waves was predicted by younger age, lower household income, children living in the household, and treatment for mental health difficulties. Of the adults recruited into the C19PRC study at baseline, 57.4% (N = 1162) participated in Wave 5. The raking procedure re-balanced the longitudinal panel to within 1.5% of population estimates for selected socio-demographic characteristics. Conclusion This paper outlines the growing strength of the publicly available C19PRC Study data for COVID-19-related interdisciplinary research.