HMRC has one of the largest and most complex IT estates in the UK, and it faces a significant challenge to modernise its IT infrastructure to keep pace with changing technology. In 2020 the NAO reported that HMRC recognised 36 Senior grades include all grades from higher executive officers up to and including senior civil servants. Less than 1% of HM Revenue & Customs’ staff were senior civil servants in March 2024. C&AG’s Report, para 2.10 and Figure 7 37 Qq 38, 42 38 C&AG’s Report, para 2.24 39 Comptroller and Auditor General, Managing tax compliance following the pandemic, Session 2022–23, HC 957, National Audit Office, December 2022 40 C&AG’s Report, paras 2.24, 2.29 41 Qq 12, 42; C&AG’s Report, para 2.29 15 the state of its IT systems as a significant risk, due to the postponement of operational maintenance and systems upgrades to secure cost savings.42 The government provided additional funding in Spending Review 2020 to improve the agility, resilience and security of HMRC’s IT. Remediating legacy systems has taken HMRC longer than it had expected and is costing more.43