Brexit: How would it happen?

With just over a week to go until the referendum on the UK's continuing membership of the European Union, the polling evidence suggests the race is "too close to call".

If the decision goes the way of 'Leave', how will the Government and Parliament go about taking the UK out of the EU?

The legal process that would govern 'Brexit' is Article 50 of the Treaty on European Union, which provides that "any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements". To invoke Article 50, a Member State must formally notify the European Council of its intention to withdraw from the EU.

There would be no set deadline following a 'Leave' vote by which the UK government would have to notify the European Council under Article 50, and indeed a delay before giving notice would leave more time for 'pre-negotiation' discussions with other Member States before the formal timetable noted below kicked in. However, in a speech to the House of Commons on 22 February 2016, the Prime Minister indicated that he would trigger Article 50 "straight away". There may not necessarily have to be a separate vote in Parliament to authorise that. It therefore seems likely that the Council would receive notice of the UK's intention to leave the EU in short order after the 23rd.

Once Article 50 had been triggered, formal negotiations would begin with the aim of concluding an agreement between the UK and the EU "setting out the arrangements for [UK] withdrawal, taking account of the framework for its future relationship with the Union". That agreement would include, among other things, the date on which the UK's exit would take effect. If no agreement could be reached, Article 50 provides that the UK would automatically withdraw from the EU two years after handing in its notice unless all the other Member States agreed to extend the deadline. Two years seems an ambitious timetable so, as long as relationships had not completely broken down, an extension could not be ruled out.

The reference in Article 50 to the agreement "taking account of the framework for its future relationship with the Union" could be crucial. As Article 50 has never been used before, it is not clear whether the withdrawal agreement would itself set out the terms of that future relationship or would simply be an 'agreement to agree' - i.e. it would set out the principles on which a future relationship should be based, without going into detail or necessarily binding the parties to anything. While it would make logical sense for the new relationship terms to take effect seamlessly upon Brexit, having to agree every point of detail in advance would inevitably put strains on the timescale.

While this may seem like a procedural detail, the distinction could be very important. Any withdrawal agreement would have to be agreed by a qualified majority under Article 238(3)(b) (at least 72% of Council members - i.e. Member States - comprising at least 65% of the EU's population), as well as the European Parliament.

By contrast, if a UK-EU trade agreement had to be agreed separately it is quite possible that that agreement would require the unanimous consent of all EU member states, particularly if it concerned issues dealt with at Member State rather than EU level. Unanimity would certainly be required if EU Treaty changes would be needed. A unanimous agreement would very likely need even more time to negotiate and implement, particularly as some Member States may need any new arrangements to be approved by their national parliaments or even in referendums. Unanimity would of course also increase the ability of individual Member States to play 'spoiler' in the hope of extracting concessions particular to their national interests. Any proposal to extend the two-year Article 50 timetable, which as noted would also require unanimity, may also give any intransigent Member States an opportunity to make their agreement conditional on substantive concessions by the UK.

Article 50 does not contain any provision for a notice of exit to be withdrawn, so it does appear to be something of a one way street. However, it also does not preclude that possibility, so if the negotiations were to head in a direction that, for example, held out the prospect of continued formal EU membership but on significantly different terms (as hinted at by Boris Johnson, among others), it is not impossible that the withdrawal notice could itself be withdrawn. From a political perspective, however, that would surely be impossible without approval in a further referendum. After withdrawal there is no fast-track back into the EU for former members - Article 50 provides that the state must follow the same Article 49 process as other applicants (though of course it may be easier for a former member to demonstrate its compliance with the membership conditions).

If you want to read more about Article 50, the European Parliament has issued a helpful briefing note about the background to and provisions of Article 50.

From a domestic perspective the process of leaving the EU manages to be both simple and potentially complex. The simple part is that Parliament would repeal the European Communities Act 1972, which is the legislative basis for the UK's membership of the EU and the applicability of EU law within the UK. The potentially complex part would be tied up with the Article 50 negotiations, as Parliament would have to review those EU laws that are currently "directly effective" in the UK - i.e. they come directly from the Treaties or from EU Regulations - and decide which it wanted to replicate in domestic legislation. Retention of certain EU law requirements in the UK may be the price of trade or other concessions sought by the UK in the Article 50.

It is not impossible that, despite the terms of the Fixed Term Parliaments Act 2011, a post-Brexit government will decide that it needs a public mandate for the terms of the UK's future relationship with the EU, and engineer an early general election (this BBC article leaves out one way that could happen, which would be for the 2011 Act simply to be repealed).

If there is a 'Leave' vote on 23 June it will only be the start of a complex and potentially lengthy process, in which various issues of great importance would be up for discussion. We will continue to post on 'Brexit' issues in the period leading up to the vote on 23 June, and perhaps even more so in its aftermath.