IT’S that strange feeling of deja vu. Headlines about Theresa May and Nicola Sturgeon being on a collision course over Brexit have a familiar ring. Last year at precisely this time I recall writing about the Prime Minister and First Minister being engaged in a game of constitutional chicken. It didn’t end well for either.

Then it was over Mrs May’s refusal, as Ms Sturgeon put it, to accept the reasoning of the Scottish Government’s 2016 White Paper, Scotland’s Place in Europe. It argued that Scotland should remain in the single market after Brexit or be allowed to have a special status in the UK that allowed Scotland to keep EU borders open. Westminster was too busy to respond in detail, and just asserted that we were leaving “as one UK”, with no exemptions.

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This year, the collision, which will happen next week when both leaders meet again, is about devolution and the Westminster “power grab” under Clause 11 of the EU Withdrawal Bill. But it is still essentially about Scotland’s constitutional status after Brexit. The Scottish Government’s emergency EU (Legal Continuity) Bill, which comes before the Scottish Parliament today, would allow the Scottish Government to keep mirroring EU law indefinitely on food standards, for example, subject to affirmation by the Scottish Parliament. That’s not something the UK Government will accept.

The other big difference is that, this year, Ms Sturgeon no longer has her ultimate deterrent against Westminster encroachment: the threat of an early referendum on independence. A second referendum is off the table for the time being, regardless of what the candidates for the SNP deputy leadership appear to think. Number 10 seems quite confident that Ms Sturgeon lacks the authority to provoke another constitutional crisis and that voters are too confused to care.

The distribution of powers after Brexit seems like a classic constitutional row that academics and constitutional anoraks like to bore on about. This is unfortunate as, for once, it actually matters. As the House of Lords EU committee made clear last year, under the Scotland Act, the 111 powers repatriated from Brussels, over agricultural support, animal welfare, food safety and so on should go straight to Holyrood “by default”. Mind you, the Lords weren’t clear on how exactly this could be reconciled with Brexit.

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The UK Cabinet Secretary, David Lidington, has made clear there will be no wholesale repatriation of powers from Brussels to Holyrood. Brexit has overtaken the Scotland Act and devolution must adapt. He has proposed giving Holyrood 86 powers, while the UK would hold the whip hand over 25 of the really important repatriated powers including agriculture, fisheries and the environment; at least until common standards had been agreed.

The Scottish Government says, rightly, that this overturns a fundamental principle of the Scotland Act 1998: that powers not reserved to Westminster automatically become powers of the Scottish parliament. There’s little doubt that Clause 11 of the EU Withdrawal Act changes this. The very fact that Mr Lidington specified the powers going immediately to Holyrood is contrary to the Act, which insists that only powers reserved to Westminster are specified. Moreover, by assuming even temporary control of important matters like agriculture, environment and fisheries, Westminster is accruing new reserved powers willy nilly, beyond those specified in Schedule 5 of the Act.

Holyrood had a unique status as a quasi-federal legislature with open-ended powers, exercising sovereignty within a supposedly unitary UK. This was always a contradiction but, like the Irish border, it didn’t seem to matter while Holyrood and Westminster were jointly subject to laws emanating from Brussels. At the very least, the constitutional ambiguity is being addressed by Brexit. Westminster is asserting its sovereignty over Holyrood in a practical way by, in effect, delegating powers. This could mean that, in future, Holyrood becomes less of a parliament and more of a unit of local government like the old Strathclyde region. Some might not shed tears but that is a big change in anyone’s money,

However, this is a highly complex argument, especially in the fog of Brexit, and even the Scottish Government accepts that the UK has a point. To preserve the integrity of the UK single market, Westminster, after it is extracted from Europe, will require the power to decide on matters such as trading standards, animal welfare and food safety if they are to apply across the entire UK. If Brussels no longer decides, Westminster has to. To take the infamous chlorinated chickens, if the UK strikes a trade deal with America over agriculture imports, all parts of the UK will have to accept them, or there would be customs checks at the border to ensure that Tesco, for instance, wasn’t sneaking the noxious fowl across to Scottish supermarkets; or vice versa.

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The Scottish Government accepts the need for “common frameworks” but says they should be struck in agreement with Holyrood and not imposed from without. For all the mind-numbing complexity of the legal arguments about Clause 11, that single word “agreement” is the essence of the dispute. Mr Lidington won’t concede it because it might effectively give Holyrood a veto on trade deals in future , rather as the Belgian state of Wallonia was able to hold up the CETA trade deal between the EU and Canada last year. The Scottish Government wants to continue to uphold environmental standards set by the EU while the UK administration wants a clean break.

And so Mrs May and Ms Sturgeon are on a collision course again. Westminster can reject the Continuity Bill and impose the EU Withdrawal Bill on Holyrood, specified powers and all. But that is the nuclear option and the MPs and Lords may baulk at precipitating such a crisis. Or will Mrs May back down and accept that agreement is the only way forward? She didn’t last time, and she’s showing little sign of doing so now. The next seven days of intense eye-balling could decide the fate of Holyrood – and Brexit.