Background By regulating behaviour at household level, COVID-19 restrictions drastically altered relationships. Given strong links between intimate relationships and health, we investigated how the pandemic impacted relational and sexual aspects of steady relationships in Britain in the 4-months following first national lockdown (23/3/2020). Methods 6,657 participants aged 18–59 years completed a web-panel survey questionnaire between 29/7–10/8/20. A quasi-representative population sample was achieved via quotas and weighting. We analysed sexual activity by age, gender and cohabitation status, and used descriptive statistics and logistic regression to explore self-perceived changes in sex and relationship quality among those in steady relationships (n=4,271). Results Of the full sample, 64.2% were in a steady relationship, mostly cohabiting (88.8%). Following lockdown, 48.9% of those in cohabitating relationships and 36.4% in non-cohabiting relationships reported sex (anal/vaginal/oral) at least weekly. Frequency of sexual activity varied by age, gender and cohabitation status. The majority reported no change in their sex life and relational quality compared with the months pre-lockdown. Among those perceiving change, quality of sex life was more commonly reported to deteriorate, whereas quality of relationship was more commonly reported to improve. Change – both positive and negative – was more commonly reported by younger people. Overall, 7% reported deterioration to a ‘lower quality’ relationship, with deterioration more commonly reported by those: in mid-life (35–44 vs. 45–59) (men, AOR:2.31; 95%CI:1.45–3.66; women, AOR=1.63; 95%CI:1.03–2.56); living in an urban area (among men) (AOR:2.61; 95%CI:1.15–5.90); and not living with a partner (among women) (AOR:2.01; 95%CI:1.28–3.16). Deterioration was associated with poor health and with decline in sexual aspects of the relationship. Conclusion COVID-19 led to an early net gain in relationship quality but net loss in quality of sex lives in steady relationships in UK. A sizeable minority of steady relationships were adversely affected with implications for sexual – and wider – wellbeing.