CULTURAL nationalists in Scotland have long dreamed of creating a political alliance of the Celtic fringe with Ireland, Scotland, Wales, even the Isle of Man and Cornwall coming together to realise a common destiny. History and geography have prevented this dream becoming a reality, at least outside folk music networks such as Celtic Connections. But thanks to Brexit, this Celtic Axis may soon become a political, and even a constitutional, fact.

The chaotic negotiations over Britain’s departure from the EU have created an unprecedented alliance of interests between Scotland, Wales and Ireland and, perhaps surprisingly, Northern Ireland. The central issue is the hard border and the attempts to prevent one being created by Britain’s departure from the customs union.

Assurances were given in last week’s phase one agreement that the entire UK would remain in the EU single market, if necessary, to prevent any border opening up, either between Northern Ireland and the UK, or between the North and the Republic. These assurances have become rather less solid over the weekend. The Brexit Secretary, David Davis, has suggested that a borderless Ireland was merely a “statement of intent” and that the guarantee given in the document was “not legally enforceable”. But the Irish will not have this commitment brushed aside or rendered problematic by the diplomatic double-speak of “nothing is agreed until everything is agreed”. The Irish Taoiseach, Leo Varadkar, is adamant that, whatever else happens, this no-border guarantee cannot be effaced by legal sophistry.

The UK Government promised in paragraph 49 that, as a backstop, it will remain in “full regulatory alignment” with the EU customs union and the single market, regardless of what happens in the phase two trade talks next year. This commitment, Mr Varadkar said, with perhaps unfortunate imagery, was “bullet proof”. More importantly, Brussels has agreed that it should be placed on a proper legal basis as soon as possible.

This promise is of enormous importance to Nicola Sturgeon. All of the parties at Holyrood are agreed, more or less, that Scotland should also retain a regulatory link with the single market, not least on free movement. Since the EU referendum, the Scottish and Welsh governments have been arguing that the devolved parliaments of the UK should have, if not a veto, then certainly a right of consultation on the Brexit process. The Scottish Government White Paper a year ago called for Scotland to remain, if not in the single market, then in full alignment with it. That was rejected out of hand by Prime Minister Theresa May.

She said that the UK would be leaving the EU as “one country” and that there could be no special arrangements for Scotland or Wales. This argument is no longer sustainable, since the UK Government has given Northern Ireland an effective veto on the Brexit process. Moreover, it has said, unequivocally, that it will act to prevent there being any hard border between Northern Ireland and the UK or between the North and the Republic. The only way to do this is for Britain to remain broadly within the rules with the customs union and the single market. Even the pro-Brexit Democratic Unionist Party seems to recognise this reality.

No one should be in any doubt about the capacity of the UK Government to wriggle out of any commitment. Albion became “perfidious” precisely because Britain has long had a reputation for reinterpreting supposedly solemn and binding agreements to its own advantage. There will be huge pressure on Mrs May to negate phase one. The fragile consensus in the UK Conservative party will not last when Brexiters realise that they’re only leaving the EU to remain in the single market and the customs union. Tory MPs like former leader Iain Duncan Smith would sooner join the Organisation of African States.

In the New Year, as the trade talks begin, it will take a determined alliance of sub-national parliaments of the British Isles to ensure that the terms of phase one are honoured, and to keep the UK in “full regulatory alignment” with the EU. Westminster must be forced to learn the hard way that the UK is no longer a unitary state, governed by a sovereign Westminster, with a nominally independent Irish appendage. The UK has always been a multinational entity but, with devolution, there has also been a diffusion of sovereignty. The Scottish and Welsh parliaments have legislative authority on devolved matters and find themselves in alignment with the Irish and the Northern Irish administrations in the effort to avoid a hard Brexit.

Thus far, the Scottish and Welsh administrations have been frustrated in their lobbying efforts by the fact that Brussels does not recognise them as EU member states. With Ireland in the lead, they could acquire a kind of diplomatic visibility, as Ms Sturgeon and Carwyn Jones, the Welsh First Minister, vigorously support the Irish Taoiseach in his attempt to force the UK to honour its promises.

Throughout the Brexit process Number 10 has sought to ignore Ireland and the border as a minor irritation. But it is the central issue in Brexit. How it is resolved will define the UK’s future relations with the EU. If the Celtic nations get their act together, they will be in the driving seat. If and when the they succeed in ensuring that the UK remains in the single market, it will be the first time that Scotland, Ireland and Wales have fully recognised their common political interests and acted upon them.

This alliance could involve more than a common bid to remain in alignment with Europe. In a curious way, this has brought Ireland closer to the UK at the same time as distancing Scotland and Wales further from it. Bringing the nations together in a renewed single market could even foreshadow a new constitutional settlement in the British Isles with a new kind of United Kingdom in which the dominant nation, England, finds itself counterbalanced by a new and cohesive Celtic alliance; and all thanks to Brexit. How’s that for unintended consequences?