Relapse to tobacco smoking for pregnant women who quit is a major public health problem. Evidence-based approaches to intervention are urgently required. This study aimed to develop an intervention to be integrated into existing healthcare. A mixed methods approach included a theory-driven systematic review identifying promising behaviour change techniques for targeting smoking relapse prevention, and qualitative focus groups and interviews with women (ex-smokers who had remained quit and those who had relapsed), their partners and healthcare professionals (N = 74). A final stage recruited ten women to refine and initially test a prototype intervention. Our qualitative analysis suggests a lack, but need for, relapse prevention support. This should be initiated by a trusted ‘credible source’. For many women this would be a midwife or a health visitor. Support needs to be tailored to individual needs, including positive praise/reward, novel digital and electronic support and partner or social support. Advice and support to use e cigarettes or nicotine replacement therapy for relapse prevention was important for some women, but others remained cautious. The resulting prototype complex intervention includes face-to-face support reiterated throughout the postpartum period, tailored digital and self-help support and novel elements such as gifts and nicotine replacement therapy (NRT).