Following the United Kingdom’s withdrawal from the Euratom Treaty, the UK Office for Nuclear Regulation, ONR, has become the UK state regulatory authority for safeguards, working with the UK Government and civil nuclear stakeholders to develop a State system of accounting for, and control of, nuclear material. A key feature of the ONR becoming the safeguards regulatory authority in the UK has been the development of an IT system, the Safeguards Information Management and Reporting System (SIMRS), with nuclear materials accountancy underpinning many safeguards activities. SIMRS provides a solution to receive, consolidate, domestic transit match, interrogate and process nuclear material accountancy reports from UK operators. Additionally, SIMRS produces reports that are required by the UK under new voluntary offer safeguards agreements with the International Atomic Energy Agency and enables reporting pertaining to UK and third-party States under nuclear co-operation agreements. This was a substantial undertaking for ONR against a highly political landscape, with challenging deadlines, multiple constraints derived from an existing extensive, complex and well established civil nuclear industry, and built from a limited initial base. SIMRS was systematically developed and tested in readiness for live operation as part of ONR’s preparations to take on its new regulatory responsibilities in respect of safeguards. This paper provides an overview of ONR’s work in developing SIMRS with details and examples relating to optioneering, procurement, testing, stakeholder engagement and management, key challenges, and learning from experience.