Objectives:Reported health behaviour change intervention attrition rates vary considerably, from 10% to more than 80%, depending on the type and setting of the treatment programme. A better understanding of the determinants of programme adherence is required. Between March and August 2020, a convenience sample of 44 individual telephone interviews, as well as 42 online Qualtrics surveys took place. The objective was to explore perceived barriers, facilitators, and opportunities for participation, sustained participation as well as initial non-participation to better understand reasons for attrition in online delivery during the COVID-19 lockdown among vulnerable and under-served groups within an Integrated Healthy Lifestyle Service (IHLS).Methods:A convenience sample of 44 individual telephone interviews, as well as 42 online Qualtrics surveys resulted in a total of 86 (33 male) individuals comprising intervention clients. Clients included children and young people (n = 16), manual workers (n = 7), Black, Asian or Minority Ethnic (n = 19), physical disability (n = 8), learning disability (n = 6), and those from areas of high deprivation (n = 19), as well as Integrated Healthy Lifestyle Service practitioners (n = 11).Results:The study revealed that more resources and support are needed for Black, Asian or Minority Ethnic; manual worker; learning disability; and high-deprivation sub-groups in order to reduce attrition rates. Specifically, a lack of technological equipment and competence of using such equipment was identified as key barriers to initial and sustained attendance among these vulnerable and under-served sub-groups during the COVID-19 lockdown.Conclusion:The pattern of differences in attrition during the COVID-19 lockdown suggests that further research is required to explore how best to ensure online health behaviour change offers are scalable and accessible to all.