By Mark Elliott
Professor of Public Law, University of Cambridge

The process whereby a Member State can leave the EU is set out in Article 50 of the Treaty on European Union. Under that process, once a Member State has notified the European Council of its intention to withdraw from the EU, the State by default leaves the EU two years later. The Miller I case arose because the UK Government, following the 2016 referendum on UK membership of the EU, asserted that it had the power — under what is known as the ‘royal prerogative’ — unilaterally to notify the European Council under Article 50, thereby setting in motion the formal legal process for exiting the EU.

That view was challenged in the case. It was argued that it would be lawful for the Government to trigger the Article 50 process only if Parliament legislated to authorise it to do so. It was further argued that the consent of the UK’s devolved legislatures would be required before Article 50 could lawfully be triggered by the UK Government.  

By a majority of eight to three, the Court held that the Government did not have prerogative power to trigger Article 50, meaning that Article 50 could not be triggered unless Parliament granted the Government power to do so (which, shortly after judgment was given in this case, Parliament did). The majority held that while the Government does have prerogative power to conduct international relations, that power could not be used to initiate a legal process that would result in Brexit.

Among the majority’s reasons for this conclusion were its views that EU law had become part of, and a source of, UK law; that its removal from the domestic legal system was not therefore a matter of international relations; that, in enacting the European Communities Act 1972 — the legislation that enabled the UK to fulfil its legal obligations when it joined the EU — Parliament had not intended the Government to be able to unilaterally remove the UK from the EU; and that the royal prerogative could not be used to accomplish a constitutional change as momentous as leaving the UK.

The dissenting judges, in contrast, considered that the 1972 Act provided only for the domestic effect of EU law for as long as the UK remained a Member State and did not preclude the use of prerogative power to accomplish Brexit. The Court was unanimous, however, that there was no legal requirement making the consent of the devolved legislatures necessary. In doing so, the Court took the view that questions of such consent were governed not by law but by a political convention.  

The judgment in this case provoked considerable controversy. That happened partly because Brexit itself was, and remains, such a divisive issue. However, the case also raised questions about whether the Court had overstepped the mark by ‘interfering’ in a ‘political’ issue. Indeed, when a lower court issued its judgment in this case (also holding that legislation would be needed to authorise the triggering of Article 50) one newspaper dubbed the judges concerned ‘the enemies of the people’, the suggestion being that the judges were seeking to undermine the will of the people as expressed in the referendum.

However, although the surrounding political context of this case was highly charged and contentious, the issue before the Court was clearly a legal one on which it was right for the Court to rule. Any legitimate controversy, therefore, relates not to whether the Court should have dealt with these issues, but to differing views on legal aspects of the constitution, including questions about the relationship between legislation and the prerogative, and about the place that EU law occupied within the domestic legal system. They are matters on which lawyers differ — as the disagreement between the majority and dissenting judges in this case demonstrates.