When​ the government decided to appeal to the Supreme Court against the High Court’s ruling that ministers could not lawfully use the royal prerogative to leave the EU without express legislative authority, many lawyers, myself included, thought it a hopeless enterprise. A court of three judges – the Chief Justice, the Master of the Rolls and Lord Justice Sales (who had been standing counsel to the government when at the bar) – had held on cogently reasoned grounds that the prior authority of an Act of Parliament was required. Nevertheless the Supreme Court sat in full, all 11 members, to hear what even the sober Constitution Unit was calling the case of the century. I began preparing this piece when the judgment was pending, pretty sure that the appeal would fail and pondering whether it would be correct to describe the outcome as something of a no-brainer. Well, the appeal did fail, and by a decisive margin of eight votes to three. But the margin conceals what was jurisprudentially a closer-run thing than the numbers suggest.

For well over four hundred years British monarchs and their ministers have contested the claims of Parliament to have the last word on matters of state. The judges have arbitrated between them, laying down as part of the common law what ministers can lawfully do in the exercise of the royal prerogative – declare war, make peace, sign treaties, grant honours, govern colonies – and what requires the authority either of the common law or of Parliament. Over these centuries it has been the rolling back of ministerial claims to arbitrary power, exercised by the use of the royal prerogative, that has shaped the British constitution.

In 1636 a London trader called Richard Chambers sued the mayor for having wrongfully imprisoned him for refusing to pay ship money. His case was that the tax was itself unlawful, having been levied by the Crown without the authority of Parliament. The court refused to hear the argument. ‘There is a rule of law,’ Justice Berkeley said, ‘and a rule of government, and things that may not be done by the rule of law may be done by the rule of government.’ It took the rest of the 17th century – a civil war, the king’s execution, the implosion of the republic, the restoration of the monarchy and the parliamentary coup d’état we know as the Glorious Revolution – to establish that government enjoyed no such extra-legal power.

In 1685 the Duke of York, who had been brought up in exile as a Roman Catholic, succeeded his brother Charles II as king and became ex officio head of the Church of England. On any view this was going to be a problem, and James II as he now was, egged on by his theological advisers, made the worst of it. Among other unwise moves he declared the Test Acts, which barred Catholics and dissenters from public office, to be of no effect, allowing him to commission Catholics as army officers at a time when the major threat to the nation was believed to come from the Catholic states of Europe. He then packed the 12-judge court which was going to decide the legality of what he was doing. Its finding in favour of a regal power to suspend or dispense with Parliament’s legislation brought a hurricane of political protest that culminated in James’s abdication and flight. In 1688 Parliament reconstituted itself and offered the crown to William of Orange on terms spelled out in a Bill of Rights that is still the foundational statute of the British state. Its second article reads: ‘That the pretended power of dispensing with laws or the execution of laws by regall authoritie as it hath been assumed and exercised of late is illegall.’ For Scotland, the Claim of Right replicated the prohibition: ‘All Proclamationes asserting ane absolute power to Cass [quash] annul and Dissable lawes … . are Contrair to Law.’

The regal authority or absolute power in question was the royal prerogative: the residue of monarchical powers by which executive government is conducted by ministers and their departments. The powers are residual for more than one reason. When in 1611 Chief Justice Coke held that ‘the King hath no prerogative but what the law of the land allows him,’ he was echoing what his predecessor Sir John Fortescue had written in the 15th century: the king had no power to alter the law (that was for Parliament) or to administer it (that was for the judges). He was also reacting to what the law reporter John Hawarde had noted in 1597: the habit of the queen’s privy counsellors – in effect her ministers – ‘to attribute to their councils and orders the vigour, force and power of a firm law, and of higher virtue and force, jurisdiction and pre-eminence, than any positive law, whether it be the common law or statute law’.

Although the ambit of the prerogative has been constricted over time, it still importantly includes the United Kingdom’s entry into and withdrawal from treaties, a function which accordingly falls to the Crown’s ministers – in substance to the foreign secretary. But because ours is a dualist system, treaties have no effect in domestic law unless and until Parliament decides to adopt them. Thus the 1950 European Convention on Human Rights had no direct effect here until 2000, when the Human Rights Act 1998 came into force. To withdraw from the convention by diplomatic act would have no effect on the legislation giving it domestic effect whereas repeal of the Human Rights Act would leave the convention without any domestic purchase.

All of this boils down to a simple proposition: to use the royal treaty-making prerogative to stultify primary domestic legislation is to do exactly what the Bill of Rights forbids – to dispense with laws by regal authority. It makes no odds whether the law concerned is the Dangerous Dogs Act, which somehow got into the argument before the Supreme Court, or the 1972 European Communities Act with its more recent add-ons: the executive cannot use prerogative power to undo what Parliament has done.

Given all this, it was hardly surprising, despite the foot-stamping of newspapers whose conception of British values seems not to include the country’s constitution, that when the prime minister let it be known that her government intended to trigger the Article 50 leaving mechanism without legislative authority, her proposal was challenged in court. The critical reasoning of the majority in the Supreme Court was this:

withdrawal is fundamentally different from variations in the content of EU law arising from further EU treaties or legislation. A complete withdrawal represents a change which is different not just in degree but in kind from the abrogation of particular rights, duties or rules derived from EU law. It will constitute as significant a constitutional change as that which occurred when EU law was first incorporated in domestic law by the 1972 Act. And, if Notice [under Article 50] is given, this change will occur irrespective of whether Parliament repeals the 1972 Act. It would be inconsistent with long-standing and fundamental principle for such a far-reaching change to the UK constitutional arrangements to be brought about by ministerial decision or ministerial action alone. All the more so when the source in question was brought into existence by Parliament through primary legislation, which gave that source an overriding supremacy in the hierarchy of domestic law sources.

This looks pretty impregnable until you turn to Lord Reed’s dissenting judgment. Standing by the principle of parliamentary supremacy, Reed argues that what Parliament enacted in and after 1972

is inherently conditional … on the UK’s membership of the EU. The Act imposes no requirement, and manifests no intention, in respect of the UK’s membership of the EU. It does not, therefore, affect the Crown’s exercise of prerogative powers in respect of UK membership … If Parliament chooses to give domestic effect to a treaty containing a power of termination, it does not follow that Parliament must have stripped the Crown of its authority to exercise that power … Withdrawal under Article 50 alters the application of the 1972 Act, but is not inconsistent with it. The application of the 1972 Act after a withdrawal agreement has entered into force (or the applicable time limit has expired) is the same as it was before the Treaty of Accession entered into force. As in the 1972 Act as originally enacted, Parliament has created a scheme under which domestic law tracks the obligations of the UK at the international level, whatever they may be … If Parliament grants rights on the basis, express or implied, that they will expire in certain circumstances, then no further legislation is needed if those circumstances occur. If those circumstances comprise the UK’s withdrawal from a treaty, the rights are not revoked by the Crown’s exercise of prerogative powers: they are revoked by the operation of the Act of Parliament itself

What the disagreement comes down to is that the majority see diplomatic withdrawal from the EU as an illicit act of the Crown draining the statutes governing EU membership from 1972 onwards of meaning and effect, while Reed (and his two supporters) sees it as one of an indefinite range of contingencies that the legislation is explicitly designed to accommodate. Using a metaphor that recurs in the judgments, the minority see the legislation as a conduit which may, and does, change repeatedly in what it carries and which may run dry for political reasons which are not the courts’ business. The majority see it as the means prescribed by Parliament by which EU law is introduced into domestic law: ‘So long as the 1972 Act remains in force, its effect is to constitute EU law an independent and overriding source of domestic law.’

From these two approaches flow either of two consequences. One, supported by the majority, is that the executive is constitutionally forbidden to stifle a statutory source of law which will continue to flow until Parliament decides otherwise. The other is that the Crown’s ministers, in conducting the UK’s foreign affairs, are free to turn off the EU tap, leaving Parliament’s legislation empty but intact.

Lord Carnwath, one of the other dissentients, based much of his agreement with Lord Reed on ministerial responsibility to Parliament as a sufficient check on executive action. Leaving aside the fact that ministers are frequently not MPs but peers and do not actually have to be members of either house (the trade union leader Frank Cousins was a rare instance), he may have forgotten what Lord Justice Farwell said in Dyson’s Case more than a century ago: ‘If ministerial responsibility were more than the mere shadow of a name, the matter would be less important, but as it is, the courts are the only defence of the liberty of the subject against departmental aggression.’

Despite Lord Reed’s astute reasoning my vote would go with the majority on the ground I began with. Since 1689 the Crown has been stripped of the power of ‘dispensing with laws or the execution of laws’. Whether diplomatic withdrawal from the EU treaties is regarded as turning off the tap or dismantling the plumbing, its purpose and effect would be to dispense with extant legislation which makes EU law part of the UK’s legal system. That is something which on principle only Parliament has authority to do.

One of the majority taking this view was Lord Sumption, whose critique of the judiciary for meddling in politics drew a good deal of attention when he was appointed directly from the bar to the Supreme Court. So Lord Reed can be forgiven his parting shot: ‘It is important for courts to understand that the legalisation of political issues is not always constitutionally appropriate, and may be fraught with risk, not least for the judiciary.’ But this case was not about the legalisation of political issues: it was about the politicisation of legal issues.

Ironically, if the government’s appeal had succeeded, another troublesome issue might have arisen: whose advice is the monarch required to take in deciding how her prerogative should be exercised? In the ordinary way, one of two things happens: either legislation spells out that it is to be one of her secretaries of state, or the cabinet, basing itself on the theory that the queen is to be advised solely by her ministers, deputes the decision to an appropriate department or takes the decision itself. But constitutionally the monarch’s proper source of advice is still the privy council, which at present consists of about 670 individuals who have held high office in the state as cabinet ministers, judges, diplomats, archbishops or whatever. We shall never know whether, had Gina Miller’s case failed, the law would have required the summoning of a body that nowadays confers in plenary session only to name the successor to the throne or (according to some) to sanction the marriage of a reigning monarch, in order to tender its collective advice as to whether the UK should leave the EU. That really would have upset the Daily Mail.

What now? The probable option is to get Parliament to pass primary legislation that delegates to ministers the power the law has so far denied them to give notice of withdrawal from the EU under Article 50. The practice of using legislation to hand ministers and their departments colossal tranches of legislative power by the use of what are known as Henry VIII clauses has been adopted with growing frequency over the last hundred years or so. The difficulty with doing it in the present situation is that it will bring down an edifice on which hundreds of other things depend, many of which are beyond the UK’s control. There may be no alternative to a detailed Great Repeal Bill and a prolonged parliamentary odyssey. And if, after two years of negotiation, no satisfactory deal has been reached with the other member states, either the UK’s membership of the EU will lapse unconditionally or our notice under Article 50 will have to be extended or withdrawn. Who will decide whether this can be done? The Court of Justice of the European Union, that’s who.

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