In the 1975 European Communities membership referendum, it was England that returned the biggest majority for the UK joining the common market: 69%. Northern Ireland’s 52% support was the smallest. Next came Scotland on 58%, though it included the only two regions in the UK that voted to stay out – the Shetland Islands and the Western Isles.

In the Brexit referendum of 2016, the political dynamics had reversed. Now voters in England and Wales voted to leave the EU, while Scotland and Northern Ireland voted to remain. This difference has set the tone for much of what has happened since. As Theresa May returns from Brussels to try and sell her withdrawal deal to parliament, the Union is straining like never before.

After the 2016 referendum came the inevitable debate about whether Scotland and Northern Ireland could avoid leaving the single market against their will. Six months after the vote, the Scottish government published a blueprint for achieving this by Scotland becoming a member of the European Economic Area after the UK leaves. The UK government refused to even entertain the idea of a differentiated Scottish Brexit.

Northern Ireland is being treated very differently. May made clear at her Lancaster House speech in early 2017 that the UK government aimed to leave the single market and customs union without returning a hard border to Ireland – after all, the Good Friday Agreement constitutionally guaranteed Northern Ireland’s right to remain in the EU.

In December of the same year, the UK and EU duly reached a political agreement that introduced the “backstop”. This envisaged that if the UK-EU trade deal did not provide for a frictionless Irish border, either Northern Ireland or the UK as a whole would remain aligned to the single market and the customs union after Brexit took place.

The new backstop

The notion of Northern Ireland remaining in parts of the single market was outrageous to many, not least the DUP. The backstop now appears somewhat differently in the “Northern Ireland protocol” codified in the draft withdrawal treaty. Barring a deal on free trade, the UK as a whole will remain in a “bare bones” customs union with the EU; while Northern Ireland will additionally remain aligned to the single market rules necessary to maintain free movement of goods across the Irish border.

Despite the shift from the original plan, this has attracted significant opposition. Prominent Brexiteers, Dominic Raab and Boris Johnson, the Scottish Conservatives and most importantly the DUP argue that it threatens the “constitutional integrity” of the UK and the Union itself.

This is a gross and needless overstatement. Northern Ireland already significantly differentiates from the rest of the UK, even in the area of protection of fundamental rights. For example, same-sex marriages are not recognised there. The laws around abortion are much more restrictive (though may need reform after the UK supreme court said earlier this year that they were incompatible with human rights law).

There are a number of cases where different parts of a member state have different relationships with the EU – Greenland is different from the rest of Denmark, for example. The UK already makes extensive use of this flexibility. Gibraltar, for instance, is outside the customs union and EU VAT area and is not part of the Common Agricultural Policy.

The sovereignty of a member state over a region has never been challenged just because EU law is applied differently there. Neither is it unique for a region to be more aligned to the EU than the rest of its metropolitan state. The Austrian territories of Jungholz and Mittelberg have been part of the EU customs territory since it was established – decades before Austria joined the EU in 1995. Meanwhile, in a special protocol of the UK withdrawal agreement, it has been agreed that another region with a constitutional relationship with the UK will remain in the EU customs territory – the UK Sovereign Base Areas in Cyprus.

The need for change

Notwithstanding the questionable legal validity of these arguments about the threat that the “Northern Ireland protocol” poses to the constitutional integrity of the UK, there remain serious questions about the current Union. The SNP certainly believes that a hard Brexit will boost the case for Scottish independence, while also making much of the different treatment of Northern Ireland.

Yet the UK constitution is sufficiently flexible that it could accommodate the nations’ different aspirations concerning Europe. Scotland could be allowed to tailor its own migration policy, for instance, as Quebec does in Canada. The UK government’s unwillingness to consider such possibilities flies in the face of an important lesson from the 2014 Scottish independence referendum: promising more devolved powers to Scotland near the end of the campaign helped to deliver the decision to stay in the Union.

The reality is that Brexit is an unprecedented challenge to the idiosyncratic UK constitution. Scotland’s inability to influence the withdrawal agreement has revealed important weaknesses around how the nations relate to the centre. The mechanisms for the intergovernmental cooperation between London and Belfast, Cardiff and Edinburgh should be strengthened to take into account the aspirations and positions of all UK constituent nations.

Brexit also needs to urgently respect the legislative autonomy of the devolved regions and avoid what has been called a “Westminster power grab”. Finally, we need to dispense with any notion that Northern Ireland remaining in the EU single market is a threat to its place in the Union.

There is an inherent contradiction in the uber-unionist position as expressed by the likes of the DUP and certain members of the Conservative Party. Unless they actively respect the differentiation that devolution introduced, the nations might conclude that secession is the most appropriate way to exercise their right for self-determination.

As Prince Tancredi Falconeri said in the classic 1950s novel The Leopard: “Everything must change so that everything can stay the same.”

Nikos Skoutaris, Senior Lecturer in European Union Law, School of Law, University of East Anglia.

This article first appeared on The Conversation.