Article 50 of the Treaty of the European Union is certainly generating its fair share of headaches. The latest has been brought on by a ruling by the High Court of Justice in England that says the British government cannot invoke the article, triggering Brexit negotiations, until it has held a parliamentary vote. Article 50 opens with:

Any member state may decide to withdraw from the European Union in accordance with its own constitutional requirements.

But, crucially, the UK does not have a written constitution in the way many countries do. It is uncodified and quietly evolving. So its “constitutional requirements” are open to interpretation.

Can the government make the decision to withdraw from the EU by itself, using a set of historic – and narrowing – powers it exercises on behalf of the Crown – the so-called “royal prerogative?” Or must the sovereign parliament be involved in that decision?

Does the royal prerogative power cover the decision to withdraw from the EU? The argument for that is that the conduct of international relations – and the making of treaties – has traditionally been seen to fall within the prerogative. But has that power been limited, or even extinguished altogether, to be replaced by powers wielded by parliament?

Landmark Brexit Ruling: What happened and what happens next?

Then comes the question of the devolved nations of Wales, Scotland and Northern Ireland. The evolving nature of British constitutional order means it is far from clear as to whether the devolved parliaments can be excluded from having a say in the triggering of the process which is set out in the rest of Article 50.

The judgment from the high court in London ruled that the government’s royal prerogative powers cannot be used to launch the Article 50 withdrawal process. That’s because doing so would lead to the removal of rights currently available to people in UK law.

These rights derive from EU law, but are made available to UK citizens because of, and through a UK statute – the European Communities Act 1972. Only parliament has the power to remove the rights that it itself has granted.

A twist in the tale

However, the London decision is not the only judgment to be made on the triggering of Article 50. Shortly beforehand, the Belfast high court ruled in essentially the opposite direction. It said neither the Westminster parliament, nor the Northern Ireland assembly, had to be asked for their consent before the British government triggers Article 50.

The Belfast court was looking at a slightly narrower set of issues, but crucially, it approached the question of how the process of notification of the decision to leave under Article 50 works in a different way from the high court in London.

The high court has worked on the basis that once the Article 50 withdrawal process is triggered by notification by the UK, it can’t be stopped. Under this view, the decision to trigger Article 50 will necessarily and directly lead to the UK leaving the EU and to people having their legal rights removed. So parliament needs to be involved in the decision to trigger Article 50, because that decision has the sort of legal consequences that only the parliament can be responsible for.

In Belfast, however, a different view was taken. The Belfast court agreed that changes in the law and to people’s rights needs to be enacted by parliament, but it said that the decision to notify the EU of the intention to leave under Article 50 does not itself generate these legal effects. Instead, the decision to trigger Article 50 is just a first procedural step which can be made by the government.

While the negotiations that will follow the Article 50 being launched will lead to changes down the line – depending on the agreement reached between the UK and the EU – nothing in law changes until then. And, when those changes do come, the process must be controlled by parliament.

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The fact that the two courts took different positions on Article 50 presents the UK’s supreme court with something of a dilemma when it comes to decide on an appeal on the high court decision in December. The supreme court will have to choose which interpretation of Article 50 it prefers – one that sees it as unstoppable, or one where the bullet can be put back in the gun. Is London or Belfast right?

But this is at least in part a matter of EU, rather than UK, law. And if there is any doubt over what an element of EU law means, the UK court must seek an authoritative ruling from the Court of Justice of the European Union in Luxembourg.

Delaying Brexit?

For the British government, the most immediate problem with all of this is that these decisions will eat into the short time remaining before the prime minister’s self-imposed March 2017 deadline for notification. Whatever the outcome of a supreme court appeal, parliamentary involvement will be necessary at some point in the withdrawal process, whether sooner or later. Another loss for the government will make that sooner.

But if the Westminster parliament needs to be involved, perhaps through a full act of parliament, where does that leave the other parliaments and assemblies of the UK? Under the normal run of things, there is a constitutional convention – a political understanding – that Westminster will not normally legislate on devolved matters without the express consent of the devolved parliaments.

The reason the Belfast court ruled against the need for devolved parliamentary involvement was because the conduct of international relations isn’t devolved. But you could also argue the European Union membership is not simply a matter of international relations, since so much of what it means to be a member affects domestic – and indeed devolved – issues. The Belfast court even acknowledged that a court in Scotland might take a different view on this matter.

In recent weeks, the complexity of the task ahead has began to become apparent, as the UK gears up to leave the EU. It just got a lot more complicated.

A version of this article has also been published by The UK in a Changing Europe