ObjectivesThe COVID-19 pandemic and its related restrictions have affected attendance to and delivery of UK sexual healthcare services (SHS). We surveyed the impact on sexual behaviour of men having sex with men (MSM) to inform future SHS provision.MethodsWe conducted a cross-sectional, anonymous, web-based survey among HIV-negative MSM at high risk of HIV infection who attended 56 Dean Street, a sexual health and HIV clinic. The survey was conducted over a 7-day period in August 2020. Data on sociodemographic characteristics, sexual behaviour and related mental well-being experienced during lockdown (defined as 23 March–30 June 2020) were extracted. Categorical and non-categorical variables were compared according to HIV pre-exposure prophylaxis (PrEP) use.Results814 MSM completed the questionnaire: 75% were PrEP users; 76% reported they have been sexually active, of which 76% reported sex outside their household. 75% reported fewer partners than prior to lockdown. Isolation/loneliness (48%) and anxiety/stress (27%) triggered sexual activity, and 73% had discussed COVID-19 transmission risks with their sexual partners. While 46% reported no change to emotions ordinarily experienced following sex, 20% reported guilt for breaching COVID-19 restrictions. 76% implemented one or more changes to their sexual behaviour, while 58% applied one or more steps to reduce COVID-19 transmission during sex. 36% accessed SHS and 30% reported difficulties in accessing testing/treatment. Of those who accessed SHS, 28% reported an STI diagnosis. PrEP users reported higher partner number, engagement in ‘chemsex’ and use of SHS than non-PrEP users.ConclusionsCOVID-19 restrictions had a considerable impact on sexual behaviour and mental well-being in our survey respondents. High rates of sexual activity and STI diagnoses were reported during lockdown. Changes to SHS provision for MSM must respond to high rates of psychological and STI-related morbidity and the challenges faced by this population in accessing services.