BackgroundAlthough a world-class routine sexually transmitted infections (STI) surveillance system, England's Genitourinary Medicine Clinic Activity Dataset (GUMCAD) collects limited behavioural data, restricting insights into drivers of STI transmission. We developed a bio-behavioural enhanced surveillance tool (BBEST) comprising a short, self-administered online survey (focusing on STI-risk and health-care seeking behaviours) of sexual health clinic (SHC) attendees, informed by systematic review, qualitative research, and community engagement, and linked to GUMCAD's electronic patient records on clinical outcomes. Our aim, using mixed methods, was to assess the feasibility and acceptability of implementing BBEST among attendees and staff. Methods16 SHCs across England invited attendees to complete an online survey from May 1 to Sept 30, 2016, and recorded numbers invited and participants' clinic number to enable linkage. Survey participation rates, proportion of participants agreeing to linkage, and linkage success rate were assessed. Two of us (SW, DR) conducted face-to-face or telephone audio-recorded, semi-structured interviews with SHC staff (n=20) to explore barriers and facilitators to implementing BBEST. Data were transcribed and thematically analysed. Online consent was obtained for survey participation and linkage. National Research Ethics Service Committee South Central, Oxford C approved the study (ref 15/SC/0223). FindingsOf 7365 attendees invited, 6283 (85·3%) agreed to participate, of whom 4585 (73·0%) logged in and consented to survey participation. Recruitment success varied considerably among SHCs, which staff attributed to differences in clinics' culture of valuing research, and to structural changes in services, which negatively affected resources available for research and staff morale, and led to frequent staff turnover. 4048 (91·2%) of survey participants consented to linkage to electronic patient records, which did not vary by age or sex but was higher among men who have sex with men (MSM) than among heterosexual men (435 [95·5%] vs 3613 [88·4%], p=0·002), and higher in white than in black Caribbean participants (2181 [93·8%] vs 568 [87·1%], p=0·059). Linkage was achieved for 3596 (88·9%) of consenting participants. InterpretationImplementing BBEST in SHCs was feasible and acceptable to staff and attendees, including among MSM and black Caribbean populations, who bear a disproportionate STI burden. Linking routinely collected surveillance data with in-depth behavioural surveys provides intelligence on the drivers and context of STI risk, strengthening the evidence-base for designing and delivering effective public health interventions. FundingNational Institute for Health Research Health Protection Research Unit (NIHR HPRU) in Blood Borne and Sexually Transmitted Infections at UCL in partnership with Public Health England and in collaboration with the London School of Hygiene & Tropical Medicine.