This study investigates the sustainability disclosure effects of the introduction of the Companies Act 2006 Regulations 2013 in the United Kingdom. The regulation mandates the disclosure of information on greenhouse gas emissions, gender distribution and human rights issues. We examine two research questions: first, whether firms increased disclosure on the mandated topics after the regulation became effective relative to a control group, and second, whether a potential increase in disclosure is moderated by firms’ reporting incentives, namely, firms’ capital market visibility, growth orientation, governance structure, prior voluntary sustainability disclosure levels and critical media coverage. Our sample consists of the FTSE-350 firms and a matched control group of US firms. We use textual analysis to assess the disclosure of the mandated sustainability topics in firms’ annual reports. Specifically, we examine two types of disclosure, namely, the disclosure of the mandated key performance indicators and the narrative disclosure. Our results reveal a significant increase for both types of disclosure relative to the control group. Overall, this treatment effect tends to be smaller for firms with higher reporting incentives, i. e., reporting incentives mitigate the regulatory effect. Taken together, our results suggest that both standards and reporting incentives shape firms’ sustainability disclosure level.