INTRODUCTION: Smoking during pregnancy is harmful to unborn babies, infants and women. Nicotine replacement therapy (NRT) is offered as the usual stop-smoking support in the UK. However, this is often used in insufficient doses, intermittently or for too short a time to be effective. This randomised controlled trial (RCT) explores whether a bespoke intervention, delivered in pregnancy, improves adherence to NRT and is effective and cost-effective for promoting smoking cessation. METHODS AND ANALYSIS: A two-arm parallel-group RCT was conducted for pregnant women aged ≥16 years and who smoke ≥1 daily cigarette (pre-pregnancy smoked ≥5) and who agree to use NRT in an attempt to quit. Recruitment is from antenatal care settings and via social media adverts. Participants are randomised using blocked randomisation with varying block sizes, stratified by gestational age (<14 or ≥14 weeks) to receive: (1) usual care (UC) for stop smoking support or (2) UC plus an intervention to increase adherence to NRT, called 'Baby, Me and NRT' (BMN), comprising adherence counselling, automated tailored text messages, a leaflet and website. The primary outcome is biochemically validated smoking abstinence at or around childbirth, measured from 36 weeks gestation. Secondary outcomes include NRT adherence, other smoking measures and birth outcomes. Questionnaires collect follow-up data augmented by medical record information. We anticipate quit rates of 10% and 16% in the control and intervention groups, respectively (risk ratio=1.6). By recruiting 1320 participants, the trial should have 90% power (alpha=5%) to detect this intervention effect. An economic analysis will use the Economics of Smoking in Pregnancy model to determine cost-effectiveness. ETHICS AND DISSEMINATION: Ethics approval was granted by Bloomsbury National Health Service's Research Ethics Committee (21/LO/0123). Written informed consent will be obtained from all participants. Findings will be disseminated to the public, funders, relevant practice/policy representatives, researchers and participants. TRIAL REGISTRATION NUMBER: ISRCTN16830506. PROTOCOL VERSION: 5.0, 10 Oct 2023.