We described the longitudinal changes in sexual behaviour and associated factors among newly diagnosed with HIV men who have sex with men participating in a prospective observational study from a London HIV clinic (2015-2018). Participants self-completed questionnaires at baseline, months 3 and 12. Information collected included socio-demographic, sexual behaviour, health, lifestyle and social support. Trends in sexual behaviours over one year following diagnosis and associated factors were assessed using generalized estimating equations with logit link. Condomless sex (CLS) dropped from 62.2% at baseline to 47.6% at month-three but increased again to 61.8% at month-12 (p-trend = 0.790). Serodiscordant-CLS increased between month-three and month-12 (from 13.1% to 35.6%, p-trend < 0.001). The prevalence of serodiscordant-CLS with high risk of transmitting to their partners at month-three was 10.7%. CLS was higher among men who reported recreational drug use (adjusted Odds Ratio (aOR) 3.03, 95%CI 1.47-6.24, p = 0.003), those with undetectable viral load (aOR 2.17, 95%CI 1.22-3.84, p = 0.008) and those who agreed with a statement "condoms are not necessary when HIV viral load is undetectable" (aOR 3.41, 95%CI 1.58-7.38, p = 0.002). MSM continued to engage in CLS after HIV diagnosis, which coincided with U = U publications and increased throughout the study
