Comparing Constitutional Models: Separation of Powers in the US and Fusion of Powers in the UK

Abstract

This article aims to compare the division of powers in the United States and the United Kingdom by examining three institutional relationships: executive-legislative, executive-judicial, and legislative-judicial. Building on foundational studies by Walter Bagehot (1867), who coined the term “fusion of powers” to describe the British system, and Charles M. Fombad (2016), who provided a comparative analysis of separation of powers in different constitutional traditions, this article addresses the gap in comprehensive three-branch comparative studies. Using a comparative constitutional analysis of primary documents, legislative acts, and secondary sources, the analysis reveals that the American system embodies a rigid separation of powers characterized by institutional independence, checks and balances, and strong judicial review, while the British system operates on a fusion of powers where executive and legislative branches are closely intertwined through parliamentary sovereignty and judges lack power to invalidate Acts of Parliament. These findings demonstrate that the British system does not resemble the rigid separation of powers found in the United States but operates on a distinct constitutional logic rooted in parliamentary sovereignty and institutional fusion, contributing to broader understanding of how constitutional designs shape governance outcomes, accountability, and the protection of individual rights.

Keywords: Separation of Powers; Fusion of Powers; Comparative Constitutional Law; United States; United Kingdom