The UK High Court (the "Court") ruled today that the government does not have the power to trigger Article 50 for the UK to withdraw from the European Union ("EU") without first putting the matter to parliamentary vote. The decision could therefore delay the trigger of Article 50 of the Treaty of European Union ("Article 50") which the Prime Minister, Theresa May, has promised to do by the end of March 2017.

The case, R (Miller) v Secretary of State for Exiting the European Union [2016] EWHC 2768 (Admin), was brought by anti-Brexit campaign group, People's Challenge, and led by investment manager Gina Miller. It was heard before the Court in October. The Court was asked to consider whether, as a matter of UK constitutional law, the government is entitled to give notice of a decision to leave the EU under Article 50 by exercise of the Crown's prerogative powers and without reference to Parliament.

The Court found that triggering Article 50 without reference to Parliament would be contrary to the fundamental rule of the UK constitution – that Parliament is sovereign and can make and unmake any law it chooses. The normal conduct of international relations and making and unmaking of treaties are taken to be matters falling within the scope of the Crown's prerogative powers. However, the Court said that this general rule exists because the exercise of such prerogative powers has no effect on domestic law, whereas withdrawal from EU by giving notice under Article 50 would inevitably change domestic law. The rights under EU law incorporated into domestic law by the European Communities Act 1972 (the "1972 Act") would cease to have effect once the UK has withdrawn from the EU.

In R v (Miller), the government argued that parliament must have intended that the Crown would retain its prerogative power to effect a withdrawal from the EU Treaties when it enacted the 1972 Act and so the Crown should now have the power to choose whether EU law should continue to have effect.

However, in handing down the judgment, Lord Chief Justice Lord Thomas, Master of the Rolls Sir Terence Etherton and Lord Justice Sales said that the Court did not accept the argument put forward by the government regarding the Crown's prerogative powers. There was nothing in the 1972 Act to support it and it was in fact contrary to the principle of parliamentary sovereignty. The judges were unanimous in their decision.

The government will now appeal the decision to the UK Supreme Court, the highest UK court with the case expected to be heard on 7-8 December 2016. The decision could therefore delay Theresa May's timetable for triggering Article 50 by the end of March. Once it has triggered Article 50, the UK will have a two year window in which to negotiate its exit from the EU.

If the Supreme Court overturns the Court's decision, then the trigger of Article 50 may well still go ahead in March and may have little impact on the current timetable. However, if the Supreme Court Justices also find that parliamentary approval must be sought before a trigger can take place, then legislation will need to be drafted before a vote occurs, making a delay possible.

Even if parliamentary approval is deemed to be required by the Supreme Court, it is not clear that the UK parliament will vote not to trigger Article 50, given that this would be going against the "will of the people" expressed in the 23 June 2016 referendum decision to leave the EU. Although the outcome of R v Miller may just delay rather than stop a Brexit, it has introduced further uncertainty into what is proving to be a highly contentious government decision to bypass parliament, with some commentators calling for an early general election next year.