Writing in the Autumn of 1938 , Louis Macneice wrote that it was fun to “draw the cork out of an old conundrum/ And watch the paradoxes fizz”.
That was again the position in the autumn and winter of 2016, when the Supreme Court wrestled with the conundrum of how the Secretary of State for Exiting the European Union could, as he claimed, have an ‘untrammelled’ prerogative power to enter or withdraw from treaties, and thereby alter the law, at the same time as Parliament had ultimate sovereign legislative authority.
The Secretary of State tried to persuade the Supreme Court that the Prime Minister had power to trigger withdrawal from the EU, even though this would inevitably lead to fundamental changes to national law, because the conduct of international relations on behalf of the United Kingdom was a prerogative function and did not (he said) need statutory authority to be exercised. It was only if the untrammelled prerogative was expressly cut down or removed by statutory words that it was in any way restrained. Since it was therefore inherent in the exercise of that prerogative that the executive could make or break treaties, then if those treaties created rights in national law, the corollary – on his argument – was that the executive had a prerogative power to remove those rights.
If the Crown wishes to enter into a treaty which will effect a change in domestic law, it must have statutory authority to do so, conferred by Parliament.
The Appellants’ argument was to the contrary. Parliamentary sovereignty is a fundamental principle of the UK constitution, and since the early seventeenth century Case of Proclamations (1610) 12 Co Re 74, the King “by his proclamation or other ways cannot change any part of the common law, or statute law, or the customs of the realm”.
So here was the paradox: if the executive had complete authority to make or unmake treaties, did that mean that it could after all change the law, but only by means of a treaty; or did it mean that the foreign relations prerogative was, to some extent, trammelled by the requirement that the executive would need legislative authority, if by entering or withdrawing from the treaty, the executive would thereby change domestic law?
The Supreme Court resolved this particular conundrum by reference to the explanation of the dualist principle in the Tin Council case (JH Rayner (Mincing Lane) Ltd v Department of Trade & Industry  2 AC 418 per Lord Oliver at 500:
“As a matter of the constitutional law of the United Kingdom, the Royal Prerogative, while it embrace the making of treaties, does not extend to altering the law or conferring rights upon individuals or depriving individuals of rights which they enjoy in domestic law without the intervention of Parliament. Treaties, as it is sometimes expressed, are not self-executing. Quite simply, a treaty is not part of English law unless and until it has been incorporated into the law by legislation. So far as individuals are concerned, it is res inter alios acta [ie something done between others] from which they cannot derive rights and by which they cannot be deprived of rights or subjected to obligations; and it is outside the purview of the court not only because it is made in the conduct of foreign relations, which are a prerogative of the Crown, but also because, as a source of rights and obligations, it is irrelevant.” (emphasis added).
In other words, there is a causal link between the two constitutional principles, first, that the prerogative cannot be used to change national law and second, that international and national law are separate and that international law is not part of national law. The only reason the Crown has power to make treaties which will not attract scrutiny of the domestic courts is because such treaties are not a source of domestic law. If the Crown wishes to enter into a treaty which will effect a change in domestic law, it must have statutory authority to do so, conferred by Parliament. Otherwise, it acts outside the scope of the prerogative powers it has been afforded. The same principle apples to terminating or withdrawing from treaties, “as a matter of logic and practical necessity” if, by doing so, the Crown will inevitably effect a change in domestic law. (Supreme Court judgment at ).
The consequence in the Miller case was that the minister could not notify the EU of an intention to withdraw in the absence of statutory authority to do so, since only Parliament has the constitutional authority to make the fundamental changes to the existing law and to existing legal rights that will necessarily follow from that decision.
only Parliament has the constitutional authority to make the fundamental changes to the existing law and to existing legal rights
It is at least arguable that the consequences go rather further. The Ponsonby Convention (confirmed in 1924, and described in Miller at ) already meant that ministers would ordinarily lay treaties before both Houses of Parliament at least 21 days before they were ratified, to enable Parliamentary objections to be heard before a decision was taken to ratify. That convention became a legal requirement by virtue of section 20 of the Constitutional Reform & Governance Act 2010 (‘CRAG’) . However, the CRAG procedure does not mean that ministers require full Parliamentary approval to enable them to ratify a treaty. As a matter of statutory construction, the UK Government could ratify a treaty even if one or both Houses of Parliament had rejected it, provided only that they had had a chance to consider it and it had not been the subject of a negative resolution in the House of Commons.
The re-statement of the old dualist principle in Miller may have incrementally expanded it. The international legal system has changed, and many Treaties (such as, for example, the UN Convention on the Rights of Disabled Persons) are at least arguably intended to create directly enforceable public law rights for individuals against states. The distinction between ‘traditional’ treaties and new generation human rights treaties of this kind was discussed by Lord Kerr in SG v Secretary of State for Work & Pensions  UKSC 16 at -, albeit in a dissenting judgment. In relation to such treaties, which may be a source of individual rights, or at least legitimate expectations, one wonders if the CRAG procedure would any longer be enough to permit ratification, or whether for treaties of that kind, express statutory authority would now be required to ratify.
No constitutional actor can assume that Parliament will change the law in future to give effect to executive intentions
Equally, since the exercise of prerogative power is now judicially reviewable, albeit on a limited basis, might it not be unlawful for the government to exercise the prerogative power to ratify a treaty imposing international obligations to change national law without clear Parliamentary authority to do so? The Supreme Court said in Miller at  that where, in that case, a referendum result ‘required’ changes to the law of the land and statute had not provided for that change, the change in the law must be made in the only way in which the UK constitution permits, namely through Parliamentary legislation. But authority for the referendum itself to be held was given by the EU Referendum Act 2015; and in any event the requirement to change the law as a result was a political, not a legal, imperative.
What if, by ratifying a treaty, the executive put the UK in a position where a change to its national law was consequently required to give effect to the international obligations to which, as a matter of international law, the UK was now committed? No constitutional actor can assume that Parliament will change the law in future to give effect to executive intentions (see Miller at ). So it seems unlikely, in the light of Miller, that the executive could now lawfully ratify a treaty which would require it to change national law, without being sure that Parliament would give it authority to do so. It would therefore seem that from now on, prior statutory authority is required to enable the executive to ratify a treaty which would require the UK to undertake implementing measures in national law. Any other approach would mean that the executive could lawfully risk placing the UK on a collision course where, in the absence of post-hoc Parliamentary ratification, it could be placed immediately in breach of its international law obligations.
It is arguable that this approach was in fact recognised, as a matter of constitutional convention, even before Miller. For example, no EU Treaty imposing legal obligations upon the UK to treat it as a source of national law has ever been ratified without prior statutory authority.
A further consequence of Miller is that statutory authority is now likely to be required before any decision to denounce or withdraw from a treaty which will effect changes to national law, unless the power to denounce or withdraw has already been provided for by or under a statute. This consequence arises as a result of the Supreme Court’s observations on the ‘conduit’ theory of legislation.
The Supreme Court held that, as a point of constitutional principle, the exercise of prerogative power could not change domestic law. But, it was said, that did not mean that the exercise of prerogative powers was always devoid of domestic legal consequences. Sometimes, it would be inherent in a particular prerogative power that it can affect the legal rights and duties of others: for example, by deciding upon the terms of service of its servants, or power to destroy property for defence purposes in time of war. In other cases, exercise of prerogative powers might change the facts to which the law applied. For example, in Post Office v Estuary Radio Ltd  2 QB 740, the Crown’s exercise of its prerogative to extend UK territorial waters meant that radio broadcasts from ships in the extended area which had previously been lawful became unlawful. However, the Supreme Court drew a distinction between that situation, where the exercise of prerogative power changed the facts to which the law applied, and situations where the exercise of the prerogative created or changed a law. (-)
The Secretary of State’s approach was to adopt Professor John Finnis’s argument that the European Communities Act 1972 (‘ECA’) itself authorised the exercise of prerogative power from time to time to alter the effect of European Union law in national law. It was, he said, a ‘conduit’ statute, which gave the government power decide what the law of the Treaties would be, including ‘turning off’ the source of EU law by exercising the prerogative power to withdraw from the EU Treaties.
The Supreme Court rejected that argument on the basis of construction of the ECA itself. It held that it was the ambit of the rights and remedies provided for by or under the EU Treaties which were incorporated into domestic law through the ‘conduit’ of section 2 of that Act which could vary from time to time; not the EU Treaties themselves. The sources of the law (the Treaties) were those statutorily added to the list in section 1(2) of the Act, all of which had Parliamentary authority. The 1972 Act did not contemplate or authorise the abrogation of EU law altogether, by the UK government deciding to withdraw from the EU treaties by prerogative act without prior Parliamentary authorisation. In other words, once EU law as defined by the Treaties had become part of UK law through the conduit of the ECA, there was no prerogative power to change the sources of EU law (and national law derived from it) by withdrawing from Treaties.
Once rights have been acquired by virtue of a treaty, it is no longer just ‘a thing done between others’ which is irrelevant in national law.
The same logic, as a matter of language, would apply to the Human Rights Act 1998 (HRA). The rights under the European Convention on Human Rights have been ‘given further effect’ in national law, but ‘the Convention’ is defined as ‘the Convention as it applies to the United Kimgdom’, and the UK has a right, in international law, to withdraw from the Convention by giving six months’ notice. If the ‘conduit’ theory were right, the government could have emptied the HRA of content simply by exercising the royal prerogative so that, after six months, the Convention did not apply to the UK and so there would be no relevant Convention rights. On that theory, the HRA could be rendered a dead letter without requiring Parliament to decide to repeal it. But, just as Parliament passed the ECA premised on a policy of UK membership of the EU, the HRA was passed on the basis of a Parliamentary premise that the UK would be a member of the Council of Europe. It would be contrary to the intention of the statute for the UK government simply to give the Council of Europe notice that it intends to withdraw and thereafter to withdraw in exercise of the Royal Prerogative, because this would alter domestic law (and remove fundamental rights protected by it). Following Miller, however, and recognising the difference between altering the rights under the Convention from time to time and withdrawing from the Convention, statutory authority would therefore now be required if a UK government evinced a policy intention to withdraw from the Convention, before it could put that policy into effect.
Arguably this principle now applies to the withdrawal from any treaty the effect of which has been to create consequential changes to domestic law rights. If, as the result of entering a treaty, new rights or indeed legal obligations have been created in national law, then – on the basis of Miller – it would be contrary to the principle of Parliamentary sovereignty for the government to take any step which would have the effect of removing those rights or obligations irrespective of whether Parliament agreed to repeal them. Once rights have been acquired by virtue of a treaty, it is no longer just ‘a thing done between others’ which is irrelevant in national law. Its alchemical effect has been to change national law, and consequently, legislative authority from Parliament is required to undo that change.
Sometimes, the force of a particular treaty will be expressly limited by the terms of the statute which gives effect to its terms. The court in Miller recognised that it would have been open to Parliament to provide expressly that the constitutional arrangements and the EU law rights introduced by the ECA should themselves only prevail from time to time and for so long as the UK government did not decide otherwise. But that is not what the statute said. This made the analogy with double taxation treaties which had been heralded in some of the academic writing and was adopted in the Secretary of State’s written submissions an unconvincing one (see ). In relation to those treaties, Parliament had foreseen that they might be changed from time to time and had expressly stated that arrangements agreed in a double taxation treaty at international level would have effect in national law only if those arrangements were specified in an Order in Council. Parliament had therefore expressly arranged for the circumstances in which such a treaty might cease to have effect in national law. If it had not done so, then it is strongly arguable that even a double taxation treaty could not be revoked or withdrawn by UK prerogative act in the absence of Parliamentary authority.
The consequence of Miller therefore goes beyond Brexit. What was intended as a reaffirmation of fundamental and long-standing constitutional dividing lines may actually have hardened them, and have had a limiting effect on the scope of the prerogative power both to enter treaties and to leave them. Arguably, prior statutory authority is now required before the executive can ratify a Treaty which is intended to have effect on national law, and only treaties which operate purely on the level of international relations can now be entered into on the basis of section 20 of CRAG. Equally, once an international treaty has had some effect in national law, the government cannot renounce or withdraw from such a treaty without prior statutory authority. It cannot now be assumed that Parliament will fall in line with the government’s intended policy; or that foreign relations can be conducted on the assumption that it will.
Helen Mountfield QC was lead counsel for the People’s Challenge (Pigney & Others), the crowd-funded parties in the Miller case, instructed by John Halford of Bindmans.
 Autumn Journal, Faber
 That provision did not apply to EU law, because treaties amending the EU Treaties in a way which affected the powers of the EU had already to be approved by an Act of Parliament, and – in some cases also by a referendum (see European Union (Amendment) Act 2008 and the European Union Act 2011.