The date on which the UK leaves the EU is an international matter. Once the UK triggered Article 50 of the TEU, notifying the EU of its intention to leave, the date on which Brexit occurred would be determined within the UK-EU relationship and governed by the terms of Article 50.

Under TEU Article 50(3), “The (EU) Treaties shall cease to apply to the State (which has notified its intention to withdraw) ... from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification ... unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.”

In other words, the UK leaves the EU on 29 March 2019, two years after it notified the EU of its intention to leave, unless either:

• a ratified and in-force Withdrawal Agreement with the EU provides for a coming-into-force date for itself other than 29 March; or

• in the absence of a Withdrawal Agreement, the UK and the remaining 27 EU Member States agree that the EU Treaties will cease to apply to the UK on a date later than 29 March.*

The date on which the UK leaves the EU is distinct conceptually from ‘exit day’ in UK law.

‘Exit day’ is a defined legal term established in the EU(W)A. It does not determine the date on which the UK leaves the EU, but, rather, functions as the trigger date for a raft of changes to take place within the UK legal order, whenever the day is determined to be.

Most importantly, as Swee Leng Harris of The Legal Education Foundation and the Bingham Centre for the Rule of Law set out for us in April 2018, under the EU(W)A ‘exit day’ is the day in UK law on which:

• the 1972 European Communities Act (ECA) is repealed (EU(W)A Section 1);

• UK law which was made previously under the ECA, as delegated legislation to implement not-directly-applicable EU law, is saved under the EU(W)A – that is, ‘exit day’ is the day on which the EU(W)A ‘switches in’ as the source of such UK law to replace the ECA, ensuring legal continuity despite the ECA’s repeal (Section 2);

• EU law which was previously directly applicable is retained as UK law (Section 3);

• the jurisdiction of the Court of Justice of the EU (CJEU) in the UK comes to an end;

• a two-year countdown starts to the expiry of the government’s power under Section 8 of the EU(W)A to amend UK law in order to "prevent, remedy or mitigate" "deficiencies" in retained EU law arising from EU withdrawal; and

• the government’s Section 9 power to make SIs to implement the Withdrawal Agreement expires.

But, beyond the EU(W)A, to take the top three results generated randomly from a search on ‘exit day’ on legislation.gov.uk on 18 March 2019, ‘exit day’ is also, for example, the day on which amendments to 10 SIs on seed marketing come into force in Scotland under the Seed and Propagating Material (EU Exit) (Scotland) (Amendment) Regulations 2019; the day on which amendments to the 1998 Competition Act, the 2002 Enterprise Act and 11 other Acts of Parliament come into force under the Competition (Amendment etc.) (EU Exit) Regulations 2019; and the day on which amendments to UK and EU Regulations on energy and eco-design product labelling come into force under the Ecodesign for Energy-Related Products and Energy Information (Amendment) (EU Exit) Regulations 2019.

To take a more high-profile example, under the Trade Bill ‘exit day’ is the cut-off point for the treatment of UK trade agreements: assuming that the Trade Bill achieves Royal Assent, the powers it gives ministers to implement UK trade agreements may be used only for agreements with countries and blocs which had signed a trade agreement with the EU before ‘exit day’.

The advantage of using a trigger date in the way in which ‘exit day’ is used is that the date only needs to be defined in law once, but can then operate consistently across multiple pieces of legislation that reference it.

While the day on which the UK leaves the EU and ‘exit day’ in UK law are conceptually distinct, they need to be the same date.

This is because of the role of ‘exit day’ as the date on which the ECA is repealed.

The ECA was required by, and gave effect to, the UK’s EU membership.

If the ECA were to be repealed before the UK leaves the EU, the UK would be in breach of its international legal obligations, under the EU Treaties, to be giving effect to EU law as long as it remains a Member State.

Equally, in the theoretically possible scenario in which ‘exit day’ in UK law were amended to fall later than 29 March but the UK left the EU on 29 March anyway, legal chaos would ensue because of the unclear status of the EU Treaties with respect to the UK.

‘Exit day’ in UK law can therefore only be amended when the date on which the UK is leaving the EU is known.

Any amendment to ‘exit day’ needs to have force by 29 March, otherwise the current 29 March definition will apply.

And ‘exit day’ does need to be defined in law as a specific, alternative date, rather than the 29 March definition simply being removed, because of the many other legal provisions - some of which are already in force - which rely on the ‘exit day’ concept.

Section 20(1) of the EU(W)A - which defines ‘exit day’ - came into force on the day the Act gained Royal Assent i.e. 26 June 2018, under Section 25 of the EU(W)A (which itself also came into force on Royal Assent).

Section 20(1) of the EU(W)A defines ‘exit day’ as a specific date: 29 March 2019 at 11.00pm.

It was unusual and controversial for a specific date to be defined in primary legislation (i.e. an Act of Parliament) in this way: it would have been more usual just to give ministers a power to use an SI to make the definition.

The original EU (Withdrawal) Bill would have followed this practice. However, at committee stage during passage of the Bill in December 2017, the government successfully moved two amendments:

• one to insert the 29 March date into the Bill, which now stands as Section 20(1) of the Act (amendment 381, carried on division by 319 to 294); and

• one, moved as a compromise after an initiative from the government backbencher Sir Oliver Letwin MP, to give ministers a delegated power to use an SI to amend the definition of ‘exit day’ in the Act, “to ensure that the day and time specified in the definition are the day and time that the Treaties are to cease to apply to the United Kingdom” if “the day or time on or at which the Treaties are to cease to apply to the United Kingdom in accordance with Article 50(3) of the Treaty on European Union is different” from the definition of ‘exit day’ in the Act (i.e. 29 March at 11pm) (amendment 400, agreed without division, now Section 20(4) of the Act).

Under the EU(W)A (Section 14 of Schedule 7), the SI to amend ‘exit day’ is subject to the draft affirmative procedure.

Use of the draft affirmative procedure means that the SI cannot take effect unless and until it has been approved by both Houses of Parliament.

There are three key stages to the procedure: the minister lays the draft SI before Parliament; it is debated and approved by both Houses; and, if it is approved, the minister may ‘make’ (sign) the SI.

The SI may come into force on or after the day it is made.

As with all SIs, the ‘exit day’ SI will not be amendable: the approval decisions in each House are on a ‘take it or leave it’ basis.

In the House of Commons, debates on affirmative SIs normally take place in a Delegated Legislation Committee (DLC). However, affirmative SIs may also be debated on the floor of the House.

We would expect the ‘exit day’ SI to be debated on the floor of the House of Commons:

• SIs debated in DLCs must still be approved by the whole House on a separate day, whereas, when an affirmative SI is debated on the floor of the House, the approval decision can take place immediately after the debate.

• Given the significance of the ‘exit day’ SI, a debate on the floor of the House may be more appropriate than a DLC debate.

• The backlog of debates that must be held on affirmative SIs is the government’s greatest challenge in managing its pre-Brexit SI programme. After the week commencing 18 March, 29 draft affirmative Brexit SIs still need to be scheduled for their House of Commons debate, and setting up and holding DLCs takes time. Even though a delay to Brexit will take some pressure off the SI programme, it will probably still be quicker for the government to find time on the floor of the House.

Whether it takes place in a DLC or on the floor of the House, as a proceeding under an Act the debate on the ‘exit day’ SI in the House of Commons will be of no more than 90 minutes (unless the government were to secure the House's approval for setting aside Standing Order 16(1) in order to provide for a longer debate).*

In the House of Lords, affirmative SIs may be debated in Grand Committee or on the floor of the House, and there is no limit to the length of the debate.

Under Standing Order 72, a motion to approve an affirmative SI may not be moved in the House of Lords until a report on the SI has been laid by the Joint Committee on Statutory Instruments (JSCI).

The JCSI asks the government to lay an affirmative SI before Parliament by the Monday of the week preceding the week in which the Committee will consider it at its weekly Wednesday meeting. In other words, under the JSCI’s normal requested scrutiny timelines, the ‘exit day’ SI would have to be laid today (18 March) for the Committee to be able to consider it at its meeting on Wednesday 27 March.*

However, the JCSI can consider SIs under an expedited procedure.

No such 'scrutiny reserve' applies in the House of Commons, which in procedural terms could thus debate the ‘exit day’ SI before the JSCI has reported on it.

In the House of Lords, the Secondary Legislation Scrutiny Committee (SLSC) also normally considers SIs, but this Committee could also move speedily if it wished to report on the 'exit day' SI, and SLSC consideration is anyway not a requirement before the Lords' decision.*

If ‘exit day’ cannot be amended until after the UK has reached an agreement on extension with the EU, but the amendment must apply by 29 March, time will be of the essence.

‘Exit day’ may not be amended by an SI made under the ‘urgency’ procedure available under Section 5 of Schedule 7 of the EU(W)A.

Beyond the JCSI’s preferred scrutiny timeline, the draft affirmative procedure is not subject to any particular deadlines or scrutiny periods.

Normally, from laying to making the SI, the draft affirmative procedure takes around six weeks.

However, given the steps outlined above, the process could be accelerated. We see no insuperable procedural obstacle to proceedings on the 'exit day' SI being completed by 29 March if the draft SI were laid, for example, on Friday 22 or Monday 25 March.*

In theory, two factors could represent potential difficulties for approval of the ‘exit day’ SI: the House of Lords’ control over the scheduling of its own business; and the possibility that the SI could be rejected.

However, the ‘exit day’ SI would be introduced in implementation of the expressed position of the elected House, in its 14 March resolution.

Given this, it is hard to see the House of Lords seeking to use its control over its own timetable to frustrate the SI’s timely approval.

As regards rejection, the House of Commons has rejected 11 SIs since 1950, and the House of Lords six. This equates to less than 0.01% of the SIs considered.

And in the Commons, as noted at the start of this post, the motion to approve the government’s plan to request an Article 50 extension was carried by 412 votes to 202. As long as the agreed extension were in line with the 14 March resolution, therefore, it would seem reasonable to assume that a majority to approve the SI implementing the extension ought to be available.*

This post was published at the end of the day on Monday 18 March. Some minor subsequent changes have been made for clarification in response to inquiries. Paragraphs containing such changes are marked .*

Thanks to Joel Blackwell, Ruth Fox and Swee Leng Harris for their assistance in the preparation of this post.