Background The COVID-19 pandemic has been a devastating and enduring mass-bereavement event, with uniquely difficult sets of circumstances experienced by people bereaved at this time. However, little is known about the long-term consequences of these experiences, including the prevalence of Prolonged Grief Disorder (PGD) and other conditions in pandemic-bereaved populations. Methods A longitudinal survey of people bereaved in the UK between 16 March 2020 and 2 January 2021, with data collected at baseline (n=711), c. 8 (n=383), 13 (n=295) and 25 (n=185) months post-bereavement. Using measures of Prolonged Grief Disorder (PGD) (Traumatic Grief Inventory), grief vulnerability (Adult Attitude to Grief Scale), and social support (Inventory of Social Support), this analysis examines how participant characteristics, characteristics of the deceased and pandemic-related circumstances (e.g. restricted visiting, social isolation, social support) are associated with grief outcomes, with a focus on levels of PGD. Results At baseline, 628 (88.6%) of participants were female, with a mean age of 49.5 (SD 12.9). 311 (43.8%) deaths were from confirmed/suspected COVID-19. Sample demographics were relatively stable across time points. 34.6% of participants met the cut-off for indicated PGD at c. 13 months bereaved and 28.6% at final follow-up. Social isolation and loneliness in early bereavement and lack of social support over time strongly contributed to higher levels of PGD, whilst feeling well supported by healthcare professionals following the death was associated with reduced levels of PGD. Characteristics of the deceased most strongly associated with lower PGD scores, were a more distant relationship (e.g. death of a grandparent), an expected death and death occurring in a care-home. Participant characteristics associated with higher levels of PGD included low level of formal education and existence of medical conditions. Conclusion Results suggest higher than expected levels of PGD compared with pre-pandemic times, with important implications for bereavement policy, provision and practice now (e.g. strengthening of social and specialist support) and in preparedness for future pandemics and mass-bereavement events (e.g. guidance on infection control measures and rapid support responses).