question: BACKGROUND: The COVID-19 pandemic has affected sexual and reproductive health (SRH) service use and unmet need, but the impact is unknown. We aimed to determine the proportion of participants reporting sexual risk behaviours, SRH service use and unmet need, and to assess remote sexually transmitted infection (STI) testing service use after the first national lockdown in Britain. METHODS: We used data from the National Surveys of Sexual Attitudes and Lifestyles (Natsal)-COVID cross-sectional, quasi-representative web survey (Natsal-COVID Wave 1). Adults aged 18–59 years who resided in England, Scotland, or Wales completed the survey between July 29 and Aug 10, 2020, which included questions about the approximate 4-month period after announcement of the initial lockdown in Britain (March 23, 2020). Quota-based sampling and weighting were used to achieve a quasi-representative population sample. Participants aged 45–59 years were excluded from services analysis due to low rates of SRH service use. Among individuals aged 18–44 years, we estimated reported SRH service use and inability to access, and calculated age-adjusted odds ratios (aORs) among sexually experienced individuals (those reporting any sexual partner in their lifetime) and sexually active individuals (those reporting any sexual partner in the past year). Unweighted denominators and weighted estimates are presented hereafter. FINDINGS: 6654 individuals had complete interviews and were included in the analysis. Among 3758 participants aged 18–44 years, 82·0% reported being sexually experienced, and 73·7% reported being sexually active. 20·8% of sexually experienced participants aged 18–44 years reported using SRH services in the 4-month period. Overall, 9·7% of 3108 participants (9·5% of men; 9·9% of women) reported being unable to use a service they needed, although of the participants who reported trying but not being able to use a SRH service at least once, 76·4% of participants also reported an instance of successful use. 5·9% of 1221 sexually active men and 3·6% of 1560 sexually active women reported use of STI-related services and 14·8% of 1728 sexually experienced women reported use of contraceptive services, with SRH service use highest among individuals aged 18–24 years. Sexually active participants reporting condomless sex with new partners since lockdown were much more likely to report using STI-related services than those who did not report condomless sex (aOR 23·8 [95% CI 11·6–48·9]) for men, 10·5 [3·9–28·2] for women) and, among men, were also more likely to have an unsuccessful attempt at STI-service use (aOR 13·3 [5·3–32·9]). Among 106 individuals who reported using STI testing services, 64·4% accessed services remotely (telephone, video, or online). Among 2581 women aged 25–59 years, 2·4% reported cervical screening compared with an estimated 6% in a comparable 4-month period before the pandemic. INTERPRETATION: Many people accessed SRH care during the initial lockdown; however, young people and those reporting sexual risk behaviours reported difficulties in accessing services and thus such services might need to address a backlog of need. FUNDING: Wellcome Trust, The Economic and Social Research Council, The National Institute for Health Research, Medical Research Council/Chief Scientist Office and Public Health Sciences Unit, and UCL Coronavirus Response Fund.