This dataset consists of 126 semi-structured (including 5 unstructured interviews) and a public survey to explore current understandings of legal sex/gender and attitudes towards its decertification. Decertification is used in this project to mean that people would no longer have a legal sex or gender (birth certificates, for instance, would no longer register a baby’s sex). Interviews were mostly conducted in-person prior to covid. After March 2020, they were undertaken via online platforms. The transcripts include interviews with a wide range of stakeholders exploring the implications of reform to legal sex and gender certification. Interviews also addressed organisations’ current practice in relation to the use of sex and gender categories, their response to gender identities such as agender and nonbinary, the challenges that innovation in this area face, and the question of how an identity-based approach to gender could combine with one attentive to structural gender inequalities. Predominantly semi-structured interviews were conducted with 1) Members of different publics using tailored interview methods to explore continuity, change and disruption in understandings and interpretations of gender (and its relationship to sex) across social and legal contexts; 2) public bodies, service providers, NGOs, regulatory bodies, religious communities, trade unions, legislative drafters, academics, and others working in related fields. A number of interviews were carried out for this research project that have not been archived. This is for several reasons, including in a few cases technological failure. However, one recurrent reason for non-archiving relates to the inability of organisational interviews to be sufficiently anonymised and the currently contentious nature of gender/ sex law reform discussions. In some cases, despite giving initial permission to archive when the interview was carried out, interviewees subsequently requested that their interview not be archived. The dataset also consists of a survey which explores wider public perceptions of reforming legal sex and gender. The ‘Attitudes to Gender’ survey was conducted as part of the ‘implications for the wider public’ strand of the Future of Legal Gender research project and focused on asking questions to gain a better understanding of what legal sex and gender status means for people, and whether it matters to individuals in their everyday lives. The survey ran from October to December 2018 in partial overlap with the UK Government’s public consultation on potential reform of the Gender Recognition Act 2004 (GRA) in England and Wales. We chose to develop the survey questionnaire ourselves rather than use pre-existing measures so we could ensure the survey mapped well onto the overall aims and objectives of the project. Sampling was opportunistic. We received 3101 usable responses to the survey. Some demographic data was collected and analysed (e.g. age, class, ethnicity, sexual preference, religion) but removed from this SSPS data set for anonymity purposes.