BACKGROUND: Perinatal mental health difficulties affect up to 27% of birthing parents during pregnancy and the first postnatal year, and if untreated are associated with difficulties in bonding and long-term adverse outcomes to children. There are large evidence gaps related to psychological treatment, particularly in group therapy approaches and parent-infant interventions. One intervention showing preliminary efficacious findings and user acceptability is Circle of Security-Parenting (COS-P), which is a brief, weekly, group programme. However, these studies were underpowered and predominantly non-randomised, and there has never been a research trial in England or with birthing parents experiencing severe and complex perinatal mental health difficulties. The aim of the research is to conduct a randomised control trial to test whether COS-P will reduce perinatal mental health symptoms in birthing parents accessing NHS perinatal mental health services, compared to treatment as usual (TAU). Secondary objectives include exploring whether the intervention improves parenting sensitivity, emotion regulation skills, attachment security and infant development. Additionally, the project aims to examine whether the intervention is acceptable to parents and NHS staff, and whether it is cost-effective. METHODS: COSI is an individually randomised, single-blind parallel arm controlled trial with an embedded internal pilot aiming to recruit 369 participants in a 2:1 ratio (intervention: TAU). Participants will be recruited from ten NHS community perinatal mental health services in England and screened based on clinical levels of both mental health symptoms (average CORE-OM score ≥ 1.1) and postnatal bonding difficulties (total PBQ score ≥ 12). This trial has 90% power to detect a MCID of 5 points on the CORE-OM. Primary and secondary outcomes will be measured at baseline, 3, 7 and 12 months after baseline. Service use and quality of life measures will also be collected alongside a process evaluation of parents' and interveners' views and experiences. DISCUSSION: This will be the first large pragmatic trial to test whether COS-P is effective for birthing parents with severe and complex perinatal mental health difficulties in improving their mental health symptoms. If shown to be effective, the intervention could be delivered widely across the NHS and other similar services globally. TRIAL REGISTRATION: ISRCTN, ISRCTN18308962. Registered 18 February 2022.