Background People with serious mental illnesses (SMI) such as schizophrenia often also have physical health illnesses and interventions are needed to address the resultant multimorbidity and reduced life expectancy. Research has shown that volunteers can support people with SMI. This protocol describes a feasibility randomised controlled trial (RCT) of a novel intervention involving volunteer ‘Health Champions’ supporting people with SMI to manage and improve their physical health. Methods This is a feasibility hybrid II randomised effectiveness-implementation controlled trial. The intervention involves training volunteers to be ‘Health Champions’ to support individual people with SMI using mental health services. This face-to-face or remote support will take place weekly and last for up to 9 months following initial introduction. This study will recruit 120 participants to compare Health Champions to treatment as usual for people with SMI using secondary community mental health services in South London, UK. We will measure the clinical and cost effectiveness including quality of life. We will measure the implementation outcomes of acceptability, feasibility, appropriateness, fidelity, barriers and enablers, unintended consequences, adoption and sustainability. Discussion There is a need for interventions to support people with SMI with their physical health. If this feasibility trial is successful, a definitive trial will follow to fully evaluate the clinical, cost and implementation effectiveness of Health Champions supporting people with SMI. Trial registration ClinicalTrials.gov, registration no: NCT04124744.

Background People diagnosed with a serious mental illness have worse physical health and lower life expectancy than the general population. Integration of mental and physical health services is seen as one service development that could better support this. This protocol describes the evaluation of the provision of a Virtual Physical Health Clinic (VPHC) and Consultant Connect (CC) services to one UK-based mental health Trust. Methods Prospective, formative, pragmatic evaluation using both quantitative and qualitative techniques and driven by implementation science theoretical frameworks. The VPHC and CC are described along with the methodology being used to rapidly evaluate their implementation, effectiveness and potential economic impact in order to inform future roll out. We will assess the implementation process through quantitative data on uptake and reach and through self-reported data to be collected from interviews and the use of validated implementation outcome assessment measures. We will assess implementation strategies using the Expert Recommendations for Implementing Change (ERIC) strategies as a framework. We will assess the health economic impact of both services using established health economic methods including cost comparison scenarios and health service utilisation analyses. Discussion Supporting the physical health management of people in psychiatric inpatient units is important in improving the physical health of this population. Integration of mental and physical health can help this to happen effectively. This initiative provides one of the first service evaluation protocols of its kind to be reported in the UK at the time of the COVID-19 pandemic.