Background The grief of relatives of patients who died of COVID-19 in an intensive care unit (ICU) has exacted an enormous toll worldwide. Aims To determine the prevalence of probable prolonged grief disorder (PGD) at 12 months post-loss and beyond. We also sought to examine circumstances of the death during the COVID-19 pandemic that might pose a heightened risk of PGD, and the associations between probable PGD diagnosis, quality of life and social disconnection. Method We conducted an observational, cross-sectional multicentre study of the next of kin of those who died of COVID-19 between March 2020 and December 2021. Participants were recruited from ICUs in South-East London. The Prolonged Grief Disorder Scale (PG-13-R), Quality-of-Life Scale (QOLS) and Oxford Grief-Social Disconnection Scale (OG-SD) were used. Results A total of 73 relatives were recruited and assessed, all of them over a year after their loss. Twenty-five (34.2%; 95% CI 23.1–45.4%) relatives of patients who died in the ICU met the criteria for PGD. Those who met the criteria had significantly worse quality of life (QOLS score mean difference 26; 95% CI 17–34; P < 0.001) and endorsed greater social disconnection (OG-SD score means difference 41; 95% CI 27–54; P < 0.001). Conclusions The findings suggest that rates of PGD are elevated among relatives of patients who died of COVID-19 in the ICU. This, coupled with worse quality of life and greater social disconnection experienced by those meeting the criteria, suggests the need to attend to the social deprivations and social dysfunctions of this population group.