This paper explores the impact of the UK's Modern Slavery Act (2015) on the disclosure of the FTSE100 companies. It conducts a content analysis of modern slavery disclosures in the modern slavery statements, annual reports, and sustainability reports from 2013 to 2019. By utilising the framework by the Business & Human Rights Resource Centre, Practical Guidance by the Home Office and the Global Reporting Initiative, we assess the extent and quality of modern slavery disclosures. Our analysis reveals a high level of compliance to the Act's minimum disclosure requirements. We also note an increase in the extent and quality of disclosures following the introduction of the Act, although quality remains low throughout the period as symbolic disclosure is predominantly in evidence. Drawing on normativity theory we identify intrinsic and contextual conditions for norm development. We find that while the presence of intrinsic conditions has positively contributed to the extent and quality of some disclosure themes, the lack of controllability and communication with suppliers has undermined related disclosure provision and constitutes a major hurdle for improving accountability in supply chains.