The Helping Families Programme is a psychoeducational parenting intervention that aims to improve outcomes and engagement for parents affected by clinically significant personality difficulties. This is achieved by working collaboratively with parents to explore ways in which their emotional and relational difficulties impact on parenting and child functioning, and to identify meaningful and realistic goals for change. The intervention is delivered via one-to-one sessions at weekly intervals over a period of 16 weeks. This protocol describes a two-arm parallel RCT in which consenting parents are randomly allocated in a 1:1 ratio to either the Helping Families Programme plus the usual services that the parent may be receiving from their mental health and/or social care providers, or to standard care (usual services plus a brief parenting advice session). The primary clinical outcome will be child behaviour. Secondary clinical outcomes will be child and parental mental health, parenting satisfaction, parenting behaviour and therapeutic alliance. Health economic measures will be collected on quality of life and service use. Outcome measures will be collected at the initial assessment stage, after the intervention is completed and at 6-month follow-up by research staff blind to group allocation. Trial feasibility will be assessed using rates of trial participation at the three time points and intervention uptake, attendance and retention. A parallel process evaluation will use qualitative interviews to ascertain key-workers' and parent participants' experiences of intervention delivery and trial participation. The results of this feasibility study will determine the appropriateness of proceeding to a full-scale trial.