Purpose: To investigate how horizontal collaboration aids organisations in responding to modern slavery legislation and in gaining a socially sustainable competitive advantage. Design/methodology/approach: Action research has been conducted in the textiles & fashion industry and a relational perspective adopted to interpret five collaborative initiatives taken to tackle modern slavery (e.g. joint training and supplier audits). The primary engagement has been with a multi-billion pound turnover company and its collaborations with 35 brands/retailers. A Non-Government Organisation (NGO) and a trade body have also participated. Findings: Successful horizontal collaboration is dependent on both relational capital and effective (formal and informal) governance mechanisms. In collaborating, firms have generated relational rents and reduced costs creating a socially sustainable competitive advantage, as suggested by the relational perspective. Yet limits to horizontal collaboration also exist. Research limitations/implications: The focus is on one industry only, hence there is scope to extend the study to other industries or forms of collaboration taking place across industries. Practical implications: Successful horizontal collaborative relationships rely on actors having a similar mind-set and being able to decouple the commercial and sustainability agendas, especially when direct competitors are involved. Further, working with non-business actors can facilitate collaboration and provide knowledge and resources important for overcoming the uncertainty that is manifest when responding to new legislation. Social implications: Social sustainability improvements aim to enhance ethical trade and benefit vulnerable workers. Originality/value: Prior literature has focused on vertical collaboration with few prior studies of horizontal collaboration, particularly in a socially sustainable supply chain context. Moreover, there has been limited research into modern slavery from a supply chain perspective. Both successful and unsuccessful initiatives are studied, providing insights into (in)effective collaboration.