Article 50: how do we leave the EU?

Voting to leave the European Union was the easy bit – years of political and legal wrangling and protracted trade deals now lie ahead, says Simon Wilson.

How does the process begin?

The formal process of leaving the European Union starts when a member state notifies the European Council of its intention to do so, under the now-famous Article 50 of the Lisbon Treaty. That has never happened before, and the lack of detail in the five-clause, 250-word Article suggests that no one in Brussels ever thought it would be needed.

What the Article says is that once notice has been served, the "Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union".

Who does the negotiating?

Legally, that agreement to leave will be concluded not with the Commission, but with the European Council (ie, the 27 remaining heads of state and government, plus the Council president and Commission president) acting with the consent of the Parliament. Once a withdrawal agreement is concluded, by a qualified majority vote (usually 20 out of 27 states) the Treaties will cease to apply to the UK, and that's it. The Article is silent on what happens if a state changes its mind. It's conceivable that the EU could come up with some sort of euro-fudge. But on the face of it, Brexit is irreversible once notice has been served.

Is there a deadline?

Yes. Once notice has been given, the departing state has two years to complete a withdrawal deal or else it gets kicked out anyway (unless all 27 other countries agree to extend the deadline). That has the effect – very much intended – of giving the EU the upper hand in the exit talks. And it's the main reason the UK is in no hurry to serve notice: it wants to conduct informal negotiations first to get some clarity on the terms of the divorce.

The withdrawal agreement will cover the untangling of shared laws, regulations and institutions; an agreement over outstanding financial obligations; and the status of the 1.3 million Britons living in the EU and 3 million EU citizens living here. Crucially, these divorce talks would not cover future trading relations (see below).

Who can trigger Article 50?

Prime Minister David Cameron thinks he can, but that view is already the subject of a legal challenge (by Mishcon de Reya acting on behalf of undisclosed clients) that will almost certainly end up in the courts.

The Article states that "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements". What, then, are the requirements for taking that decision under Britain's famously uncodified (if not quite "unwritten") constitution? During the referendum campaign, Cameron said that if the UK voted to leave, he would notify Brussels right away.

In doing so, he was assuming the right to use prerogative powers to enact the result of a referendum which, legally, was merely advisory. In the event, of course, he instead resigned and said it would be up to the next prime minister to press the button. But lawyers are divided on whether he, or Theresa May, actually has that right at all.

Why wouldn't they have the right?

Because parliament is sovereign. Under case law and constitutional precedents from the early 17th century onwards, the executive cannot take away rights given by parliament and it cannot undermine existing statutes, such as the European Communities Act 1972.

A number of authorities on constitutional law have argued forcefully in recent weeks that if a prime minister were to invoke Article 50 without an Act of Parliament giving consent, "the declaration would be legally ineffective as a matter of domestic law and it would also fail to comply with the requirements of Article 50 itself". It may of course be that political realities will trump any legal wrangling and make this question a mere formality.

How so?

The current parliament, though overwhelmingly made up of MPs who were for Remain, shows no signs of wishing to ignore the referendum result, and would presumably give such consent, if asked. Brexit means Brexit. On the other hand, the British political landscape is currently in such a state of flux that predictions made with certainty one day can look foolish the next. It is perfectly plausible that legal, constitutional and political wrangling will add significantly to the complexity and timescales of exit.

Moreover, the referendum question was silent on the terms of any exit deal. So it is even possible, argued the Tory former attorney-general Dominic Grieve this week, that if public opinion shifted against Brexit in the course of exit talks, a second referendum might become justified and/or politically unavoidable, adding vastly to the uncertainty.

Alternatively, the government might decide to formalise certain preconditions – akin to Gordon Brown's "five tests" for joining the euro – before invoking Article 50. Voting to leave was the easy bit. Actually getting out could be much, much harder.

What about the future of trade?

Crucially, negotiations over Britain's future trading relationship will be separate and (unless Britain agrees a straightforward pre-cooked arrangement such as membership of the European Economic Area, which is politically unlikely) they are very likely to take much longer than the two years fixed by Article 50. They will also require approval by more than 30 European, national and regional parliaments, raising uncertainties that will pose significant economic risks to the UK.

It is not even yet certain that the EU will agree to negotiate these matters in tandem with an exit deal: the Council president Donald Tusk has insisted they can't begin until the withdrawal agreement is in place, and would then take at least a further five years "without any guarantee of success".