Throughout the EU referendum campaign, the Remain camp pressed a very troubling claim: that a Leave vote would herald the break-up of our United Kingdom. Naturally the SNP took up this line, but so too did Carwyn Jones, the Labour First Minister of Wales, and even William Hague. Those of us who argued the counter-case were a small minority. Thus far, however, our arguments have borne up rather well. Nicola Sturgeon bet heavily on the received wisdom when she came out for a re-run of the 2014 independence referendum the morning after the Brexit vote, and the result has been only misery for her party. Meanwhile, Jones’ claims that Brexit would force Wales to “choose between two unions” were rendered ridiculous the moment the principality voted Leave and have grown no more plausible with the passage of time. However, there is a very real way in which our departure from the EU could do deep, perhaps eventually fatal harm to our country: if the withdrawal is conducted in such a way as to mangle the constitution. Which brings us to the battle over Clause 11 of the EU Withdrawal Bill. This is the row over the alleged ‘power grab’ of devolved responsibilities by Westminster. Under Clause 11 of the Bill, all powers currently exercised in Brussels will be repatriated to London by default, unless and until devolution arrangements are agreed at a later date. An improbable coalition stretching from the separatists to the Scottish Conservatives, by way of the likes of Jones and Richard Leonard, all insist that a huge range of powers currently pooled in Brussels need to be immediately passed down to the devolved legislatures once repatriated. It should surprise nobody that Jones and Sturgeon are trying to leverage a moment of crisis into an extension of their own powers, for that is their invariable response to any constitutional moment. But the active collusion of unionists is worrying for it might induce ministers, unaware of the risks, to make concessions this country cannot afford. Those looking for a more comprehensive overview should read this excellent paper from These Islands, and I have set out elsewhere how the ‘power grab’ thesis fundamentally misrepresents the basis of the dispute. The legal and moral case behind it is nonsense from top to bottom: these are not devolved powers and nobody has ever even contemplated devolving them. Relatively few people can get excited about arcane constitutional disputes, but the stakes in this one cut to the very heart of what makes our kingdom function. Here is the danger of MPs tampering with Clause 11. Ask a simple question: could we remain a member of of the Single Market if we repatriated all or nearly all of the powers it harmonises and started doing our own thing? It seems ridiculous to suggest it, especially for supporters of British membership of said Single Market. Yet pro-Remain, pro-UK politicians such as Andy Burnham and Adam Tomkins claim to believe that these powers can be passed straight from the continental to the sub-state level without jeopardising the British internal market. Perhaps they’re right, but they should have to prove their improbable case and the white heat of the withdrawal process is no time to make that call. Clause 11 is not a permanent veto on devolution, simply a provision that these powers should stay in Westminster unless and until the case for passing them on can be proven. Some might be tempted by the idea that Parliament could devolve these powers now and then cobble together “common UK frameworks” at a later date. This would be foolish. Such is the present state of British constitutional debate that no power passed down is ever withdrawn, even if it is manifestly unsuitable for devolution (such as Northern Ireland’s welfare powers). Thus, any post-hoc bid to maintain a coherent UK single market would be conducted in a series of tortuous quadrilateral negotiations in which the nationalists (big-N or small-n) heading the devolved governments would hold all the cards – and that would likely be the format for the exercise of most of these powers thenceforth. Now, Jones has been quite explicit about his desire to convert the UK from a (somewhat complicated) unitary state into a loose confederacy, but once again that is the sort of profound constitutional change which at the very least warrants measured and deliberate consideration. It would almost certainly give the SNP an endless new source of manufactured grievances whilst making the Union operate less effectively – and thus make the case for independence seem more attractive. It would be a dereliction of duty for MPs to usher in such a radical transformation by accident, for the sake of striking a blow against the Government, or in the misguided belief that they are somehow defending the status quo. Westminster is at a permanent disadvantage in constitutional disputes. Holyrood and Cardiff Bay, and the broader ecosystems of ‘civic Scotland’ and ‘civic Wales’ beneath them, are permanently-established lobbies for the expansion of their own powers. There is no equivalent force defending British-level governance, and our constitution has suffered for it. It may suffer a further blow now. But for all the bombast of the devocrats, MPs have a bullet-proof case for standing firm on Clause 11 and ensuring it passes unimpeded. If they voted Remain, the idea that these powers can be devolved without danger directly contradicts the entire foundational logic of the Single Market. If they voted Leave, it seems perverse to fight so hard to reinvigorate Parliament only to immediately undermine it again. And if they truly believe in devolution, they should be keenest of all to ensure that Parliament takes the time to implement it properly.