Legislative Sovereignty, Executive Power, and Judicial Review: Comparative Insights from Brexit

In June 2016, participants in a United Kingdom referendum voted to leave the European Union (EU) by a margin of 52% to 48%. The timing and terms of Britain’s exit (commonly known as “Brexit”) are the subject of on-going public and parliamentary debate. But the mechanism by which Brexit is to be formally commenced was clarified by the U.K. Supreme Court at the end of January 2017 in the landmark case R (Miller) v. Secretary of State for Exiting the European Union. The question presented was whether ministers of Theresa May’s government could give notice of the U.K.’s withdrawal from the EU “without prior legislation passed in both Houses of Parliament and assented to by HM The Queen.” The secretary of state argued that the power to withdraw was part of the royal prerogative exercisable by ministers without prior parliamentary action. However, in light of the far-reaching changes to domestic law that would result from terminating EU membership treaties, the court held that withdrawal “must be made in the only way in which the UK constitution permits, namely through Parliamentary legislation.”On the one hand, the Miller decision may be seen as a resounding reaffirmation of the principle of parliamentary sovereignty in British constitutional law. Notwithstanding the facts that a majority of voters supported Brexit and that government ministers ordinarily have the power to terminate treaties without legislative approval or judicial review, formal notice of withdrawal under Article 50 of the EU Treaties could not be given unless and until Parliament so agreed. Yet on the other hand, a comparative analysis of Miller also reveals some significant limitations on parliamentary power in the United Kingdom relative to congressional power in the United States—even though Congress is constrained by the Constitution and enjoys no sovereignty over the other branches of government. This Essay explores these limitations on parliamentary power and argues that legislative sovereignty is best understood not as an immutable principle laid down in Britain’s constitutional history, but rather as an evolving ideal that continues to develop to this day.