Objectives: The COVID-19 Psychological Research Consortium (C19PRC) Study aims to assess the impact of the COVID-19 pandemic in the adult population in multiple countries. This paper describes the third wave of the UK survey (the ‘parent’ strand of the Consortium) during July-August 2020. Methods: Adults (N=2025) who participated in the baseline and/or first follow-up surveys were reinvited to participate in this survey, which assessed: (1) COVID-19 related knowledge, attitudes, and behaviours; (2) the occurrence of common mental disorders; as well as the role of (3) psychological factors and (4) social and political attitudes, in influencing the public’s response to the pandemic. Weights were calculated using a survey raking algorithm to ensure that the cross-sectional sample is nationally representative in terms of gender, age, and household income, and representative of the baseline sample characteristics for household composition, ethnicity, urbanicity and born/raised in UK. Results: 1166 adults (57.6% of baseline participants) provided full interviews at Wave 3. The raking procedure successfully re-balanced the cross-sectional sample to within 1% of population estimates across selected socio-demographic characteristics. Conclusion: This paper demonstrates the strength of the C19PRC Study data to facilitate and stimulate interdisciplinary research addressing important public health questions relating to the COVID-19 pandemic.