The exceptional complexity of the legal aspects of UK withdrawal from the EU is due to the fact that it involves the interaction of three legal systems – international law, EU law and national law. The constitution of the EU is contained in treaties, taking effect under international law. EU law is made and administered by three classic organs of a constitution – legislative, executive and judicial. EU law is an external source of law integrated into the national legal systems of the member states. It can override national law in case of conflict.

It follows that the legal aspect of each stage of constructing a new legal relationship between the UK and the EU must be examined separately, in the light of law coming from the three interacting legal systems.

1. Conservative Party General Election Manifesto, May 2015, page 72

‘It will be a fundamental principle of a future Conservative Government that membership of the European Union depends on the consent of the British people – and in recent years that consent has worn wafer-thin. That’s why, after the election, we will negotiate a new settlement for Britain in Europe, and then ask the British people whether they want to stay in the EU on this reformed basis or leave. David Cameron has committed that he will only lead a government that offers an in-out referendum. We will hold that in-out referendum before the end of 2017 and respect the outcome.’

An election manifesto is not legally binding. On 20 February 2016 the Prime Minister announced that he had reached agreement with the EU on a ‘new settlement’ for Britain in Europe.

2. European Union Referendum Act 2015 (17 December 2015), sec. 1

‘(1) A referendum is to be held on whether the United Kingdom should remain a member of the European Union…  (4) The question that is to appear on the ballot papers is— “Should the United Kingdom remain a member of the European Union or leave the European Union?”’

This is the statutory basis for holding a referendum.  The Act does not specify the legal effect of the outcome of the referendum.  Section 2 identifies those entitled to vote in the referendum and, by implication, those excluded from voting.

3. Referendum (23 June 2016). Electorate: 46.5 million.  Turnout: 72.2%.  Leave: 51.9%.  Remain: 48.1%

4. Treaty on European Union (Consolidated Version, 2009), Article 50 (incorporated from the Lisbon Treaty, 2007)

(1) Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.

(2) A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, 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…

(3) The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.

Eight points should be noted.

(a) Article 50 governs the withdrawal process as a whole. It is a treaty provision under international law, and a provision of EU law, and a part of UK law.

(b) The notification to the European Council is of an intention to withdraw. It is not a notification of withdrawal.

(c) The EU Council would draw up ‘guidelines’ for the negotiation of the withdrawal agreement. They are the equivalent of ‘instructions’ to a delegation in traditional diplomatic practice. EU Council guidelines are normally very detailed, and their drafting can be a lengthy and highly animated process.

(d) The UK would not take part in drawing up the Council guidelines. To enable the Council to agree them, it must have some idea of what the future relationship of the UK to the EU might be. Article 50 says that the agreement with the EU takes account of the framework for the UK’s future relationship. This means that there would be some prior interaction between the UK and the EU before the Council can draw up the guidelines for the main negotiation, and before the actual negotiation of the withdrawal agreement can begin.

(e) Withdrawal takes place automatically two years after the date of notification, if a withdrawal agreement has not entered into force by that date, unless the EU and the UK decide otherwise.  In the case of a member state such as the UK, the EU and the UK would surely so decide.

(f) Article 50 does not say if the notification of intention to withdraw can be withdrawn at any time.  The EU Council would no doubt accept such a withdrawal of the notification.

(g) The European Court of Justice has jurisdiction over the implementation of Article 50.

5. Decision to give notification of intention to withdraw

Seven points should be noted.

(a) Article 50 says that ‘a Member State may decide…’ and ‘[that] Member State shall notify…’ (italics added). The first is a legal power. The second is a legal obligation. It is normal international practice for the executive branch of the national government to express externally a decision to exercise a power or to carry out a duty under a treaty. The process for enabling the executive branch to do so is a matter of national constitutional law and practice.

(b) In the UK, the power of the executive branch to exercise a public power may arise under an Act of Parliament, the common-law or EU law, the three sources of law in this country. Where there is no legislation conferring on the executive branch a power to give a notification of withdrawal, the authority can only arise under the common-law or EU law.

(c) The common-law recognises certain inherent inherited powers that may be exercised by the executive branch. It is no longer appropriate to refer to these powers as the Royal Prerogative, except as a way of indicating their historical origin. They can be abridged or abolished by Act of Parliament, and often have been. The courts determine their existence and their content at any given time.

(d) The expression ‘parliamentary sovereignty’ is misleading and should be avoided. The ultimate principle of the British constitution is the Rule of Law, brilliantly identified as such by A.V. Dicey in 1885 from a study of the whole course of our constitutional history. All public power is subject to the law as determined and enforced by the courts. He spoke of the ‘supremacy of Parliament’ because Acts of Parliament override the common law. Parliament does not have an inherent non-statutory legal authority over the other two leading organs of the constitution, the Government and the courts.  All three are involved in a complex system of checks-and-balances.

(e) On a number of occasions, Parliament has altered its own powers by its own act – the Bill of Rights, the Parliament Acts, the Statute of Westminster, and the European Communities Act. The courts have recognised and enforced those alterations. Acts of Parliament may give to the two Houses of Parliament powers, for example, in relation to the control of subordinate legislation, or the recently created power of the House of Commons to cause an anticipated general election.

(f) Parliament’s non-legislative and non-statutory powers are essentially political – enforcing the constitutional conventions of ministerial accountability and collective cabinet responsibility, and acting as Winston Churchill’s ‘grand forum of the nation’.

(g) A notification to the European Council of an intention to withdraw is a political act, subject to the constitutional conventions mentioned in (f) above. It does not ‘trigger’ anything. It fulfils a UK treaty obligation, enabling the European Council to begin the formal process of drawing up negotiating guidelines.

6. Negotiation of a withdrawal agreement

The negotiation of a possible withdrawal agreement has already begun.  As noted above, there are three phases of negotiation – (1) a political process within the UK to establish the general lines of the UK’s future relationship with the EU; (2) a preliminary negotiation with the European Council establishing a framework for the actual negotiation, enabling it to establish the guidelines for the formal negotiation of the agreement; (3) the negotiation of the withdrawal agreement. Steps (1) and (2) have already begun. Whether they should be continued into step (3) is a political decision, subject to the constitutional conventions mentioned in 5(f) above.

Four further points should be noted.

(a) The negotiation would be immeasurably more complicated than the accession negotiations of 1971-72. The law and structures and systems of the EU are immeasurably more extensive and complex than those of the European Communities in those days. The EU itself also now has a very large volume of treaty relations, multilateral and bilateral. There would have to be parallel negotiations to restructure the UK’s relationship to those treaties.

(b) The constitutional role of public opinion has changed dramatically in the meantime, with relentless exposure and discussion of all public affairs, in media of communication that are 24-hour and irresistible. It would not be possible to conduct the negotiations in the relatively secretive way in which the accession terms were negotiated. Even if Whitehall were determined to be as secretive as possible, leaks of negotiating positions from external sources could not be prevented.

(c) The Houses of Parliament would surely want to track the negotiations in existing committees or in newly established committees. And the devolved governments in the UK would surely want to be involved in some continuing way.

(d) International negotiators are capable of overcoming, in forms of words, any negotiating challenge. But four issues of a constitutional character seem particularly daunting. (1) In what way would the UK be involved in decisions taken by the EU institutions that seriously affect the UK? (2) To what extent would EU law remain applicable in the UK, even if its supremacy over UK law were removed? (3) Would the European Court of Justice have jurisdiction over the withdrawal agreement, given that the agreement would bind the EU institutions and the remaining member states, and given that the agreement would be a treaty between the UK and the EU governed by international law? (4) Would the International Court of Justice have jurisdiction over the agreement?

7. Implementation of a withdrawal agreement

There would be an UnEuropean Communities Bill to replace the European Communities Act of 1972. Like section 2 of that Act, its drafting would inevitably echo, by similarity rather by difference on this occasion, the terms of Henry VIII’s Act of Supremacy of 1534 which excluded the jurisdiction of the Church of Rome from Britain. These things are remarkable examples of the legislative supremacy of Parliament, in a country which does not have a written constitution.

Three points should be noted.

(a) The disentangling of UK law from EU law would be an immense task, since EU law affects almost every aspect of UK law and government and administration. Again, it would be a much more difficult task than introducing European law at the time of UK accession.

(b) The removal of EU law would alter an incalculable number of existing rights and obligations of natural and legal persons in the UK, and settled legal situations based on those rights and obligations. There would be no end to the cascade of legal consequences and of potential litigation.

(c) The legislation would have to be ready to enter into effect on or before the date of UK ratification of the withdrawal agreement or, at the very latest, on or before the date of the agreement’s entry into force.

8. Ratification of the withdrawal agreement and entry into force

In signing and ratifying treaties, the executive branch of government exercises a prerogative power relating to the conduct of the UK’s international relations. Unlike the position in most other countries, parliamentary assent is not required, beyond the passing of the implementing legislation.

On the EU side, the withdrawal agreement would be concluded by the European Council, acting by a qualified majority, after obtaining the consent of the European Parliament. A qualified majority is 55% of the member states, representing at least 65% of the population of the EU (excluding, on this occasion, the UK as a member state and its population).

Three points should be noted.

(a) Implementation of the agreement in other member states would be as difficult, if not more difficult, than in the UK, especially in those countries where constitutional amendments would be required, or where a referendum would be required.

(b) Entry into force of the agreement would presumably depend on ratification by all the other member states. It is possible for a treaty to enter into force with a lesser number of ratifications, perhaps on a provisional or partial basis.  It is difficult to see how that would be possible on this occasion.

(c) Difficulties in securing ratification in particular countries might lead to a need for partial renegotiation and amendment of the withdrawal agreement. A striking precedent is the renegotiation and amendment of part of the UN Law of the Sea Convention, designed to remove objections (so far unsuccessfully) to ratification of the Convention by the United States of America.

Conclusion. The withdrawal of the UK from the EU is a legal enterprise of unprecedented complexity, involving the exercise of legal powers under three legal systems.  The Rule of Law, as a fundamental principle of liberal democracy, means that the legality of the exercise of those powers may be determined by the laws and the courts of those legal systems.

Philip Allott is Professor Emeritus of International Public Law at Cambridge University and a Fellow of Trinity College Cambridge. His Eutopia. New Philosophy and New Law for a Troubled World was published in July 2016.

(Suggested citation: P. Allott, ‘Fundamental Legal Aspects of UK Withdrawal from the EU: Eight Stages on the Way to a New Relationship’, U.K. Const. L. Blog (9th Nov 2016) (available at https://ukconstitutionallaw.org/))