Background Tongue-tie can be diagnosed in 3–11% of babies, with some studies reporting almost universal breastfeeding difficulties, and others reporting very few feeding difficulties that relate to the tongue-tie itself, instead noting that incorrect positioning and attachment are the primary reasons behind the observed breastfeeding difficulties and not the tongue-tie itself. The only existing trials of frenotomy are small and underpowered and/or include only very short-term or subjective outcomes. Objective To investigate whether frenotomy is clinically and cost-effective to promote continuation of breastfeeding at 3 months in infants with breastfeeding difficulties diagnosed with tongue-tie. Design A multicentre, unblinded, randomised, parallel group controlled trial. Setting Twelve infant feeding services in the UK. Participants Infants aged up to 10 weeks referred to an infant feeding service (by a parent, midwife or other breastfeeding support service) with breastfeeding difficulties and judged to have tongue-tie. Interventions Infants were randomly allocated to frenotomy with standard breastfeeding support or standard breastfeeding support without frenotomy. Main outcome measures Primary outcome was any breastmilk feeding at 3 months according to maternal self-report. Secondary outcomes included mother’s pain, exclusive breastmilk feeding, exclusive direct breastfeeding, frenotomy, adverse events, maternal anxiety and depression, maternal and infant NHS health-care resource use, cost-effectiveness, and any breastmilk feeding at 6 months of age. Results Between March 2019 and November 2020, 169 infants were randomised, 80 to the frenotomy with breastfeeding support arm and 89 to the breastfeeding support arm from a planned sample size of 870 infants. The trial was stopped in the context of the COVID-19 pandemic due to withdrawal of breastfeeding support services, slow recruitment and crossover between arms. In the frenotomy with breastfeeding support arm 74/80 infants (93%) received their allocated intervention, compared to 23/89 (26%) in the breastfeeding support arm. Primary outcome data were available for 163/169 infants (96%). There was no evidence of a difference between the arms in the rate of breastmilk feeding at 3 months, which was high in both groups (67/76, 88% vs. 75/87, 86%; adjusted risk ratio 1.02, 95% confidence interval 0.90 to 1.16). Adverse events were reported for three infants after surgery [bleeding (n = 1), salivary duct damage (n = 1), accidental cut to the tongue and salivary duct damage (n = 1)]. Cost-effectiveness could not be determined with the information available. Limitations The statistical power of the analysis was extremely limited due to not achieving the target sample size and the high proportion of infants in the breastfeeding support arm who underwent frenotomy. Conclusions This trial does not provide sufficient information to assess whether frenotomy in addition to breastfeeding support improves breastfeeding rates in infants diagnosed with tongue-tie. Future work There is a clear lack of equipoise in the UK concerning the use of frenotomy, however, the effectiveness and cost-effectiveness of the procedure still need to be established. Other study designs will need to be considered to address this objective. Trial registration This trial is registered as ISRCTN 10268851. Funding This project was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment Programme (project number 16/143/01) and will be published in full in Health Technology Assessment; Vol. 27, No. 11. See the NIHR Journals Library website for further project information. The funder had no role in study design or data collection, analysis and interpretation. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.