AbstractIntroductionA high BMI during and after pregnancy is linked to poor pregnancy outcomes and contributes to long-term maternal obesity, hypertension, and diabetes. Evidence of feasible, effective postnatal interventions is lacking. This randomised controlled trial will assess the feasibility of conducting a future definitive trial to determine effectiveness and cost-effectiveness of lifestyle information and access to Slimming World® (Alfreton, UK) groups for 12 weeks commencing from 8 to 16 weeks postnatally, in relation to supporting longer-term postnatal weight management in women in an ethnically diverse inner city population.Methods/analysisWomen will be recruited from one maternity unit in London. To be eligible, women will be overweight (BMI 25–29.9 kg/m²) or obese (BMI ≥ 30 kg/m²) as identified at their first antenatal contact, or have a normal BMI (18.5–24.9 kg/m²) at booking but gain excessive gestational weight as assessed at 36 weeks gestation. Women will be aged 18 and over, can speak and read English, expecting a single baby, and will not have accessed weight management groups in this pregnancy. Women will be randomly allocated to standard care plus lifestyle information and access to Slimming World® (Alfreton, UK) groups or standard care only. A sample of 130 women is required.Feasibility trial objectives reflect those considered most important inform a decision about undertaking a definitive future trial. These include estimation of impact of lifestyle information and postnatal access to Slimming World® (Alfreton, UK) on maternal weight change between antenatal booking weight and weight at 12 months postbirth, recruitment rate and time to recruitment, retention rate, influence of lifestyle information and Slimming World® (Alfreton, UK) groups on weight management, diet, physical activity, breastfeeding, smoking cessation, alcohol intake, physical and mental health, infant health, and health-related quality of life 6 and 12 months postnatally. An embedded process evaluation will assess acceptability of study processes and procedures to women.Ethics/disseminationLondon–Camberwell St Giles Research Ethics Committee, reference: 16/LO/1422. Outcomes will be disseminated in peer-reviewed journals and presentations at national and international conferences.Trial registrationTrial registration number:ISRCTN 39186148. Protocol version number: v7, 13 August 17. Trial sponsor: King’s College London.