BREXIT, the story so far: In December, a spanner was thrown into the works when the Irish Government, and Northern Irish MPs, won a guarantee that there would be no hard border in Ireland. This was a blow to Tory MPs hoping for a hard or “clean” Brexit because the UK Government also had to agree that it would remain in “full regulatory alignment” with the EU Customs Union to the extent necessary to prevent any such border emerging.

Next week, an even bigger spanner may be tossed into the works. That is, if the Scottish and Welsh parliaments, following yesterday’s Holyrood debate, pass a Continuity Bill ensuring that they too remain, effectively, in regulatory alignment with the EU. It would be another break point. Indeed, it could mean the end of Brexit as a UK phenomenon, and its replacement with Engxit – a departure from the EU by England alone.

The Continuity Bill has been dismissed as a “nat stunt”, but it was first adopted by the Labour First Minister of Wales Carwyn Jones, before Nicola Sturgeon. It says, essentially that a devolved parliament has certain law-making powers and it has the right to pass a law ensuring that these powers will remain in place after the UK leaves the EU. That may seem a tautology. But continuity would mean that all the devolved parliaments, not just Northern Ireland, might remain within the remit of the EU single market to the extent that they are currently subject to its laws.  The Presiding Officer, Ken Macintosh, believes that such a bill is outwith Holyrood’s competence, but the Scottish Government, with the non-Tory opposition parties united, intends to proceed.

Now, given that Theresa May has insisted all along that she doesn’t want to tamper with Holyrood’s powers, and indeed wants to devolve yet more powers to Scotland and Wales, you might wonder why she doesn’t just agree  to continuity. Indeed, there’s some evidence that the UK Government had toyed with the idea of giving the Scottish Parliament guarantees that its powers are sacrosanct. It was supposed to come forward with amendments to Clause 11 of the EU Withdrawal Bill earlier this month to guarantee Holyrood’s constitutional status quo. But there were problems.

The UK Government has said all along that it will not accept parts of the UK remaining effectively in the EU after Brexit. Affirming Holyrood’s legislative rights doesn’t necessarily do that, but it might cause problems down the line. Environmental and agricultural legislation in Scotland is based on EU laws and covers things like GM crops and chlorinated chickens. If these laws were set in stone by continuity, then it might make it impossible for the UK Government to negotiate UK-wide trade deals with third parties. Continuity would also mean that Theresa May could not use her so-called “Henry VIII” powers under the Withdrawal Bill to pick and choose what powers remain in Scotland after Brexit. She insists that she doesn’t want to do that, but few in Holyrood believe her.

Indeed, even Scottish Conservative MSPs endorsed a unanimous report from Holyrood’s constitution committee this month which said that Clause 11 of the EU Withdrawal Bill was “incompatible with devolution”. It said that it would fundamentally alter the fundamental principle underlying the 1998 Scotland Act, that any powers not specifically reserved to Westminster are devolved. This was also the view of a House of Lords EU committee report last year which said that, as a matter of law, the powers currently exercised by Holyrood should automatically remain there after Brexit “by default”and that Westminster had no right to interfere.

Now you’d be forgiven for thinking that this is all technical stuff that gets academics and constitutional lawyers very excited, but doesn’t mean much in the real world. This is exactly what many in the UK Government believed about the Irish border issue. Until December 2017. When Number 10 finally looked at the practicalities, it became clear that a hard border was an unavoidable consequence of taking Northern Ireland out of the EU Customs Union. There would need to be new rules on tariffs, regulations and free movement of people. That’s what “taking back control” means. You couldn’t restore a border in the mind and try to pretend that nothing had happened on the ground.

As soon as this reality was accepted, the UK further realised that it would have to agree to remain in regulatory alignment with the Customs Union and the single market, at least to the extent necessary to prevent a hard border. And since this alignment would have to apply to the entire UK, it meant that the whole question of the UK’s departure was reopened. Brexit no longer meant Brexit, but keeping within many of Brussels’s rules. Mrs May has tried to row back from the phase 1 agreement, but it is very difficult.  The EU lawyers took her at her word and wrote full alignment for Ireland into the draft treaty document this week.

The Continuity Bill could add Scotland and Wales to the Brexit-minus equation. Now, let us be clear:  Westminster is quite within its powers to vote down any such measure. Indeed, the Supreme Court could – according to a legal opinion on the Scottish Parliament’s own website – rule it illegal, on much the same ground that it said the Scottish Parliament had no right to veto Article 50. Sovereignty, ultimately, resides with Westminster. However, that ignores the political dimension. Even many Tory MPs might be reluctant to impose legislative consent upon Holyrood. It might be difficult even to frame the motion under which any Scottish Continuity Act would be overturned.

Already, Northern Irish DUP MPs have exerted an effective veto on the Brexit process by insisting on regulatory alignment with the UK.  Many in the Lords would argue that it is unwise to risk breaking up the UK just to prevent Scotland remaining in broadly the same relationship to Europe as Northern Ireland. Moreover, if the UK is – as appeared to  be the case in December – remaining in alignment to the EU single market, how can it refuse to allow Scotland the same right?

By imposing its will over Holyrood and Cardiff, Westminster risks confirming Carwyn Jones’ claim that Brexit involves a “naked power grab” and the extinction of devolution. Throughout, the Scottish Secretary, David Mundell, has affected a world-weary cynicism with the devolution dimension. He thinks it’s all just trouble-making by the nationalists. But this was never just an SNP issue, and the trouble hasn’t begun yet.