The Government faced an interesting choice in the aftermath of the Divisional Court’s unanimous and powerful ruling that it had no Royal Prerogative power to invoke Article 50.

Seeking a resolution or some other vote of both Houses would have been a non-starter (because only another statute could overwrite the 1972 European Communities Act), but one option was to humbly accept that it had misunderstood its powers and seek Parliament’s authority with a short Bill that could have been some way through the Parliamentary process by now.

It would, of course,have been for MPs to debate the contents of that Bill – and some of the trickier issues it would need to tackle, such as the position of the devolved governments (see below).

The alternative was to‘double down’ and take up the suggestion made by Lord Leveson at last July’s directions hearing of an appeal fast-tracked straight to the UK’s Supreme Court. This is, of course, what the Government did. It may now be wondering about the wisdom of that choice, even though it gave short shrift to a plea by senior Tory heavyweights to withdraw its appeal as the hearing approached.

One reason for that is that the devolution issues first raised by the People’s Challenge group were considered but not determined by the Divisional Court but cannot now be avoided.

That is partly because of the interventions of the Welsh and Scottish Governments, but even more importantly in terms of the scope of the appeal, the decisions of the Northern Irish courts to refer five questions of law to be decided at the same time as the Government’s Miller appeal:(1) does the Northern Ireland Act 1998 read together with the Belfast Agreement (also known as the Good Friday Agreement) and the British-Irish Agreement mean an Act of Parliament is required before Article 50 can be invoked?; (2) if so, is the consent of the Northern Ireland Assembly required first?; (3) if not, does the 1998 Act read together with the Belfast Agreement and the British-Irish Agreement operate as any form of restriction on the exercise of the prerogative?; (4) does section 75 of the 1998 Act prevent the prerogative power being exercised to invoke Article 50(2) before equality consequences are assessed; and (5) does the exercise of the prerogative power without the consent of the people of Northern Ireland impede section 1 of the 1998 Act – the constitutional cornerstone of the current devolution arrangements and the mechanism for any future referendum on a united Ireland?

These are serious and important question and similar issues arise in Scotland and Wales.They are important because EU law forms part of the mortar of the UK’s devolution arrangements – allowing rights to be enforced against the devolved governments and imposing EU obligations on them,none of which could be replicated in a Great Repeal Bill. To take one example, the British-Irish Agreement creates institutions responsible for implementing EU-funded cross-border initiatives which become meaningless if Northern Ireland is part of a state no longer bound by EU law. This doesn’t mean that Northern Ireland’s special circumstances and constitutional arrangements ‘block Brexit’. But it does mean that those arrangements have to be dismantled, very carefully, by Parliament.

The Government has yet to reveal its strategy to counter these arguments but, as noted in an earlier update, it has published its written arguments (the ‘Written Case’ or ‘Printed Case’ in Supreme Court-speak) in support of its appeal. There is no new argument that the 2015 Referendum Act empowers ministers in law, nor any attempt to persuade the court that the decision on Art 50 is so“polycentric” that the courts should not decide the case (the‘hands off’ argument that got nowhere in the Divisional Court).

Instead, the ‘new and improved’ argument is that “…acts of the Government in the exercise of the prerogative can alter domestic law”, that this is possible unless Parliament has created and clearly marked out statutory ‘no go’ areas for prerogative-wielding ministers. The European Communities Act created the opposite, the Government argues:it functions as a ‘conduit’ through which the products of international negotiations at an EU level can pass into UK law and be enjoyed by UK nationals. But the conduit works both ways, it argues:if the Government decides that an international agreement should be ended and acts accordingly – here by invoking Article 50 to leave the EU – rights may be taken away through the same ‘conduit’.So EU rights are not solid, reliable or permanent in a legal sense –they are ‘ambulatory’, always in flux and dependent on what the Government of the day agrees with other states “from time to time”.

This is a staggering proposition and has implications that go far beyond this case.

Suppose the government is right. EU citizenship rights enjoyed for decades, that most UK nationals were born with, can be stripped away without any Parliamentary involvement at all. The logic of the Government’s case is that this could have been done if there had been no EU Referendum, or even if majority of votes cast had been in favour of remaining.

But this argument does not, and cannot, stop at the stripping away of EU rights. Any right that was not protected by Parliamentary ‘stop’ signs in an Act of Parliament could be stripped away just as easily from UK citizens,from common law rights to privacy or property through to rights to access the courts and confidential legal advice. The implications are more than unsettling; they are horrifying.

Fortunately, there are many reasons why this is not the law which will be set out in the People’s Challenge case later today, but here are two of the most important.

First, UK citizens do not have to show that Parliament has created a reserved area where their and others’ EU rights can be enjoyed. The right starting point is to inquire whether the Government can show any precedent for use of the prerogative to dispense with citizens’ rights. As Lord Bingham said in Bancoult(No 2) “[w]hen the existence or effect of the royal prerogative is in question the courts must conduct an historical enquiry to ascertain whether there is any precedent for the exercise of the power in the given circumstances. ‘If it is law, it will be found in our books.  If it is not to be found there,it is not law’” The ‘books’ would have to be very old ones to support the Government’s true case because the Bill of Rights was drafted to protect against precisely this kind of prerogative-based power grab – as were the Scottish equivalents,the Acts of Union.

Secondly, the European Communities Act cannot plausibly be interpreted as an executive-empowering instrument rather than a rights-granting one.Parliament did not, in some express or implied way surrender to the Crown its own power to control the scope of the EU law, and indeed the EU Treaties, from time to time. The point of the Act was to take us into the European Communities (and later the Union), not to take us in or out as the executive pleases in a game of international hokey-kokey. And as for EU law being ‘ambulatory’, the People’s Challenge case will show that it is anything but: EU rights are fundamental and irreplaceable.

Meanwhile, it has been reported that the Government faces a further challenge,this time based on the EEA agreement which, it is said, will continued to apply to the UK even if Article 50 is invoked. That is very likely right in law – the UK is a party to the agreement in its own right as well as through the EU. But by itself it is unlikely to provide much of a safety net because, like the EU, the UK can give notice and leave. The Miller appeal will likely decide how that can be done. Whether it should leave the EEA will, if the appeal fails, be for Parliamentarians, not the Courts to decide.