Regulatory discretion is a central concept in the study of the regulatory state. Yet little attention has been paid to the origins of regulatory discretion, and how it varies across polities, policy areas, and over time. This paper presents a conceptualization of regulatory discretion that draws on three dimensions: delegation, content, and procedure. It argues that to measure regulatory discretion in legislation, we need to identify provisions that delegate regulatory powers to governments and then examine the extent to which exercising regulatory powers is constrained. Using Natural Language Processing techniques, this paper presents descriptive findings of the dynamics of regulatory discretion in the United Kingdom between 1900 and 2020. The findings portray how discretion has been constrained and formalized over the years through the content of regulations, while it has still retained high levels of flexibility in exercising regulatory powers. In doing so, the findings illustrate the rise of the British regulatory state through its legislative language.