IntroductionThe prevalence of smoking is high among people living with severe mental illness (SMI). Evidence on feasibility, acceptability and effectiveness of smoking cessation interventions among smokers with SMI is lacking, particularly in low- and middle-income countries. We aim to test the feasibility and acceptability of delivering an evidence-based intervention,i.e., the IMPACT smoking cessation support for people with severe mental illness in South Asia (IMPACT 4S) intervention that is a combination of behavioural support and smoking cessation pharmacotherapies among adult smokers with SMI in India and Pakistan. We will also test the feasibility and acceptability of evaluating the intervention in a randomised controlled trial.MethodsWe will conduct a parallel, open label, randomised controlled feasibility trial among 172 (86 in each country) adult smokers with SMI in India and Pakistan. Participants will be allocated 1:1 to either Brief Advice (BA) or the IMPACT 4S intervention. BA comprises a single five-minute BA session on stopping smoking. The IMPACT 4S intervention comprises behavioural support delivered in upto 15 one-to-one, face-to-face or audio/video, counselling sessions, with each session lasting between 15 and 40 minutes; nicotine gum and/or bupropion; and breath carbon monoxide monitoring and feedback. Outcomes are recruitment rates, reasons for ineligibility/non-participation/non-consent of participants, length of time required to achieve required sample size, retention in study and treatments, intervention fidelity during delivery, smoking cessation pharmacotherapy adherence and data completeness. We will also conduct a process evaluation.ResultsStudy will address- uncertainty about feasibility and acceptability of delivering smoking cessation interventions, and ability to conduct smoking cessation trials, among adult smokers with SMI in low- and middle-income countries.ConclusionsThis is to inform further intervention adaptation, and the design and conduct of future randomised controlled trials on this topic. Results will be disseminated through peer-review articles, presentations at national, international conferences and policy-engagement forums.Trial registrationISRCTN34399445(Updated 22/03/2021), ISRCTN Registryhttps://www.isrctn.com/.