After much anticipation, the Supreme Court has given its landmark Brexit judgment.

The constitutional importance of the case is emphasised by the fact that all 11 Supreme Court Justices were involved in the case (R (Miller & others) v Secretary of State for Exiting the European Union [2017] UKSC 5. The political importance can be seen from the court's introductory observations: 'It is worth emphasising that nobody has suggested that this is an inappropriate issue for the courts to determine. It is also worth emphasising that this case has nothing to do with issues such as the wisdom of the decision to withdraw from the European Union, the terms of withdrawal, the timetable or arrangements for withdrawal, or the details of any future relationship with the European Union.'

At the heart of the case were two questions:

1) whether serving notice to leave the EU affected domestic law and domestic rights;

2) whether ministers could effect such changes by prerogative powers instead of an Act of Parliament.

By a majority of 8 to 3, the Supreme Court concluded that an Act of Parliament was required. The claimants argued that to trigger Article 50 without parliamentary approval would be contrary to UK constitutional principles, which provide that ministers cannot normally exercise prerogative powers to change domestic law without an Act of Parliament. Further, they argued it 'would frustrate the rights and duties enacted by Parliament in the European Communities Act 1972 and would be inconsistent with the object and purpose of that Act, namely to give effect to the rights and duties consequent on membership of the EU'.

The government argued that ministers had the power to make or break treaties and that in triggering Article 50, they would simply be exercising that power, as well as carrying out the will of the people. The Supreme Court held that whilst the referendum had great political significance, the legal significance is determined by the Referendum Act 2015. The Referendum Act enabled the referendum to be held but did not specify the consequences after the result was known. Accordingly, parliamentary authority was required to give effect to the decision in the usual way i.e. through an Act of Parliament.

The decision

By a majority of 8 to 3, the Supreme Court dismissed the Secretary of State's appeal. The decision means that an Act of Parliament is required to give the government authority to trigger Article 50 of the Lisbon Treaty thereby starting the process of leaving the European Union.

The Judgment is an historic decision. It is important because it confirms the dividing line between the sovereignty of Parliament and the prerogative powers of the government to make or break treaties where those treaties give rights and obligations in domestic law. The Supreme Court decided that because parliament had used its legislative powers to confer the EU rights into domestic law, the government cannot use its prerogative powers to remove those rights by triggering Article 50 thereby breaking the Treaty on the European Union.

Devolution

When the devolution Acts were passed, it was assumed that the UK would remain part of the EU but continued membership was not guaranteed and is not a devolution requirement. The devolved legislatures do not have a vote on the decision of the UK to leave the EU. The Supreme Court held that relations between the EU and the UK (including triggering Article 50) is reserved to the UK parliament and government.

What happens next?

At the time of writing, it is not known what effect leaving the EU will have on the rights of UK residents. Once triggered the UK will have a two year period to leave the EU regardless of whether an agreement with the remaining member states has been agreed. A unanimous decision of the remaining member states would be required to extend the deadline.

The government has stated that it will set out its EU Exit plans in a formal White Paper. Meanwhile in acceptance of the Supreme Court decision, a parliamentary bill seeking authority to trigger Article 50 and to begin formal negotiations with the EU is expected to be tabled in the near future. David Davis, the Secretary of State for leaving the EU stated that: '[the bill] will be the most straightforward bill possible to give effect to the decision of the people and respect the Supreme Court's judgment'. The bill is likely to pass in the Commons, particularly as a motion has already been passed by MPs supporting the government's timetable for notification. However, opposition parties have vowed to table numerous amendments to the bill as well as ensuring the process of leaving the EU is fully scrutinised.

The bill might come up against stronger opposition in the House of Lords where there is greater opposition to Brexit. It may be that the Lords are willing to derail the process despite the unelected status.

Whether or not it is possible to keep to the prime minister's deadline of 31 March to trigger Article 50 remains to be seen. As for the timetable for the negotiations which will follow, the judgment reminds us that negotiations to join the EU started in 1960 and were concluded by the signature of the Treaty of Accession some 12 years later, in 1972.