In total, 81% of women in the UK start breastfeeding, but fewer than half continue beyond 6 weeks. Peer support in the early postnatal period may encourage women to breastfeed for longer. To develop a breastfeeding peer-support intervention based on motivational interviewing (MI) for breastfeeding maintenance and to test the feasibility of delivering it to mothers in areas with high levels of social deprivation. Intervention development and a non-randomised multisite feasibility study. Community maternity services in three areas with high levels of social deprivation and low breastfeeding initiation rates in England and Wales. Pregnant women considering breastfeeding. Women who did not plan to breastfeed, who had a clinical reason that precluded breastfeeding continuation or who were unable to consent were excluded. The intervention Mam-Kind was informed by a survey of infant feeding co-ordinators, rapid literature review, focus groups with service users and peer supporters and interviews with health-care professionals and a Stakeholder Advisory Group. It consisted of face-to-face contact at 48 hours after birth and proactive one-to-one peer support from the Mam-Kind buddy for 2 weeks, followed by mother-led contact for a further 2-6 weeks. Recruitment and retention of Mam-Kind buddies, uptake of Mam-Kind by participants, feasibility of delivering Mam-Kind as specified and of data collection methods, and acceptability of Mam-Kind to mothers, buddies and health-care professionals. Nine buddies were recruited to deliver Mam-Kind to 70 participants (61% of eligible women who expressed an interest in taking part in the study). Participants were aged between 19 and 41 years and 94% of participants were white. Intervention uptake was 75% and did not vary according to age or parity. Most contacts (79%) were initiated by the buddy, demonstrating the intended proactive nature of the intervention and 73% ( = 51) of participants received a contact within 48 hours. Follow-up data were available for 78% of participants at 10 days and 64% at 8 weeks. Data collection methods were judged feasible and acceptable. Data completeness was > 80% for almost all variables. Interviews with participants, buddies and health service professionals showed that the intervention was acceptable. Buddies delivered the intervention content with fidelity (93% of intervention objectives were met), and, in some cases, developed certain MI skills to a competency level. However, they reported difficulties in changing from an information-giving role to a collaborative approach. These findings were used to refine the training and intervention specification to emphasise the focus of the intervention on providing mother-centric support. Health-care professionals were satisfied that the intervention could be integrated with existing services. The Mam-Kind intervention was acceptable and feasible to deliver within NHS maternity services and should be tested for effectiveness in a multicentre randomised controlled trial. The feasibility study highlighted the need to strengthen strategies for birth notification and retention of participants, and provided some insights on how this could be achieved in a full trial. The response rate to the survey of infant feeding co-ordinators was low (19.5%). In addition, the women who were recruited may not be representative of the study sites. The National Institute for Health Research Health Technology Assessment programme.