Were a Prime Minister to attempt to do so, the declaration would be legally ineffective as a matter of domestic law and it would also fail to comply with the requirements of Art 50.
There are a number of reasons for this. At the most general, our democracy is a parliamentary democracy, and it is Parliament, not the government, that has the final say about the implications of the referendum, the timing of an Art 50, our membership of the Union, and the rights of British citizens that flow from that membership. More specifically, the terms and purpose of the European Communities Act 1972 also support the correctness of the legal position set out above. The reason why this is so important is not only because Art 50, once triggered, will inevitably fundamentally change our constitutional arrangements, but also because the timing of the Art 50 declaration has major implications for our bargaining position with other European states.
Article 50 regulates the process of withdrawal from the Union; it is a once-and-for-all decision. If no agreement has been reached after two years, the member state is left without any deal with the EU. It is possible to extend the time period – but this requires the unanimous agreement of the EU Council.
It may be argued that implicit within the Article is a right for the member state to revoke the notice to withdraw. Yet this argument depends on reading such a right into a text from which it is conspicuously absent. The point is however probably moot since the UK must trigger Art 50 expecting and intending to exit, and it could not assume that it is able to later withdraw notification.
The question of how an Art 50 notification can be given is of paramount importance. Unfortunately, this is less clear than it might first appear. Article 50 specifies that the decision to leave the Union must be made in ‘accordance with its own constitutional requirements’ – but what are these requirements in the British system?
In his resignation speech on 24 June, David Cameron appeared to assume the royal prerogative could be used to trigger Art 50. The prerogative is widely used in foreign affairs, which Parliament has largely left in the hands of the government. The treaty-making prerogative of the crown is one such area.
If he is correct, the Prime Minister enjoys complete discretion: the trigger could be pulled in October, next year, or in ten years’ time.
The courts have long asserted the right to rule on the existence of a purported prerogative power. And one of the earliest constraints on the ambit of the prerogative was that it could not be used to undermine statute; where the two are in tension, statute beats prerogative. In one of the seminal cases of the common law, The Case of Proclamations (1610) 12 Co. Rep. 74, Sir Edward Coke declared: ‘... the King by his proclamation… cannot change any part of the common law, or statute law, or the customs of the realm…’
A recent statement of this long established principle can be found in the Fire Brigades Union Case  2 AC 513, where Lord Browne-Wilkinson stated: ‘It would be most surprising if, at the present day, prerogative powers could be validly exercised by the executive so as to frustrate the will of Parliament as expressed in a statute and, to an extent, to pre-empt the decision of Parliament whether or not to continue with the statutory scheme.’
The proposition is an important one: government cannot take away rights given by Parliament, and it cannot undermine a statute. For the courts to hold otherwise would place the rights of British citizens at the mercy of the government.
Admittedly, the precise ambit of this principle is open to different interpretations. A narrow reading would limit its application to situations where the statute prescribes how government must act, but the government circumvents that guidance by recourse to prerogative. The wider principle is that it is not open to government to turn a statute into a dead letter by exercise of the prerogative; government may not act in ways that cut across the object and purpose of statutes. In our view, the wider principle correctly states the law and is particularly apt here, as we are concerned with a constitutional statute upon which an extensive system of rights is founded.
Consequences for Art 50
As we have seen, the purpose of a member state embarking on the Art 50 process is to withdraw from the EU, with the EU treaties ceasing to apply to the UK. Can such a decision be made by the government alone, even following a referendum?
First, the European Communities Act 1972 (ECA 1972) is, as its long title states, an Act ‘to make provision for the enlargement of the European Communities to include the United Kingdom’. The long title of the Act is a permissible aid to interpreting the terms, and object and purpose of the Act.
Section 2 then provides that all such rights, powers, liabilities, obligations and restrictions from time to time created or arising by or under the EU treaties are part of UK law.
The intention of the ECA 1972 is to provide for the UK’s membership of the EU and for the EU treaties to have effect in domestic law. The purpose of triggering Art 50 would be cut across the Act and render it nugatory. Once a withdrawal agreement took effect, or if no deal was reached, the ECA 1972 would be left a dead letter. Indeed, there would not be any need for Parliament to repeal the ECA 1972 once the Art 50 process was completed because there would be no remaining rights and obligations for the UK under the terms of the EU treaties.
By issuing an Art 50 declaration, the Prime Minister would start the process that would inevitably end in the loss of EU rights.
Second, the Art 50 declaration will strip British citizens of their rights in relation to the European Parliament. The European Parliamentary Elections Act 2002 confers a right to vote and to stand in European elections. The government cannot unilaterally do an act which will render the 2002 Act nugatory and strip away the rights that it confers.
More examples could be given, but the general point is plain. Our membership of the European Union has conferred a host of legal rights on British citizens. Applying the common law principle found in The Case of Proclamations and Fire Brigades Union, and in associated case law, the government cannot remove or nullify these rights without parliamentary approval.
This has significance not only in terms of our domestic law, but also for EU law. Article 50 specifies that a decision to leave the European Union must be made in conformity to a member state’s constitutional requirements. If the Prime Minister sought to issue an Art 50 without parliamentary approval, it would not satisfy this test; it would not be effective in European law.
Role of Parliament
There appears a dilemma: how can Parliament legislate to take the UK out of the EU before the exit negotiations are complete? There is an answer: before an Art 50 declaration can be issued, Parliament must enact a statute empowering the Prime Minister to issue the declaration and empowering the government to make statutory changes necessary to bring about exit.
Is this a mere formality? The political reality might be ‘yes’. Parliament might feel bound by the referendum to pass such a statute. But the answer in constitutional terms is ‘no’. As a matter of constitutional law, Parliament is not bound to follow the referendum. A number of options are constitutionally possible.
First, it could decide not to grant this power at all. Parliament might decide that the case for Brexit has not been made – or made under a false prospectus. As Edmund Burke taught us, ours is a representative, not a direct, democracy. Those representatives whose consciences required them to reject the referendum vote would have to justify themselves to their electorates at the next general election – an event that is likely to arrive quite soon. We should make clear that we take no position as to whether Parliament should adopt such a course, but it is open to Parliament as a matter of constitutional law.
Second, Parliament could decline to invoke Art 50 whilst it is in the dark about the key aspects of the new relationship with the EU. Parliament might conclude that to require the government to issue the notice immediately would be contrary to the national interest, and could require the government to engage in negotiations to obtain some form of framework agreement before allowing Art 50 notification. This option would comply with the outcome of the referendum.
Finally, of course, Parliament could decide to authorise notice under Art 50 at once by empowering the Prime Minister to issue the declaration.
The Art 50 process raises complicated legal and political issues and is pregnant with risk. The referendum result itself does not address the question of how the UK should leave the EU. It is up to the government and to Parliament to ensure that the exit is managed consistently with the UK’s national interest.
Our analysis leads to the possibility that the process of extraction from the EU could be a very long one indeed, potentially taking years to come about. A quick pull of the Art 50 trigger is unlikely to be feasible under the UK’s constitutional arrangements and rushed negotiations may well not be desirable. Brexit is the most important decision that has faced the UK in a generation; our constitution – Parliament – gets to make this decision, not the Prime Minister.
Contributors Nick Barber, Fellow, Trinity College, Oxford; Tom Hickman, Reader in Public Law, UCL and a practising barrister at Blackstone Chambers; and Jeff King, Senior Lecturer in Law, UCL
This article is an edited version of a blog published on 27 June 2016 on the UK Constitutional Law Group Blog and is reproduced with permission.