Intimate relationships are ubiquitous and exert a strong influence on health. Widespread disruption to them may impact wellbeing at a population level. We investigated the extent to which the first COVID-19 lockdown (March 2020) affected steady relationships in Britain. In total, 6,654 participants aged 18-59 years completed a web-panel survey (July-August 2020). Quasi-representativeness was achieved via quota sampling and weighting. We explored changes in sex life and relationship quality among participants in steady relationships (n = 4,271) by age, gender, and cohabitation status, and examined factors associated with deterioration to a lower-quality relationship. A total of 64.2% of participants were in a steady relationship (of whom 88.9% were cohabiting). A total of 22.1% perceived no change in their sex-life quality, and 59.5% no change in their relationship quality. Among those perceiving change, sex-life quality was more commonly reported to decrease and relationship quality to improve. There was significant variation by age; less often by gender or cohabitation. Overall, 10.6% reported sexual difficulties that started/worsened during lockdown. In total, 6.9% reported deterioration to a "lower quality" relationship, more commonly those: aged 18-24 and aged 35-44; not living with partner (women only); and reporting depression/anxiety and decrease in sex-life quality. In conclusion, intimate relationship quality is yet another way in which COVID-19 has led to divergence in experience.