WHAT does the Brexit Bill do? Four headlines. First, it sets aside the keystone of European law in the UK, repealing the European Communities Act. Freed from the underpinning European rules and principles, ministers in Whitehall and MPs in Westminster will ultimately be able to take a hacksaw to the statute book, whether to dismantle mandatory competitive tendering for public sector contracts, or to hollow out the rights of workers.

Second, the Bill snapshots EU law on exit day. This will ensure that new EU regulations won’t apply after Brexit – but without the chaos and uncertainty which could come from pulling the rug suddenly from under the many existing European legal rules which apply in this country. This is the idea of “retained EU law”.

Thirdly, the Bill makes UK ministers the legal janitors of Brexit, giving them powers – across devolved and reserved matters – to mop up the irrational mess which is likely to be caused.

Lastly, the Bill does something dismally predictable, and something odd. Yes, we come to the devolution bit.

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The Secretary of State for Scotland, always in the loop, hinted that yesterday’s Bill would not be “a power grab, but a power bonanza” for devolved parliaments across these islands. We can only assume David Mundell was shown an earlier version of the Bill, neglected to read it, or comprehensively misunderstood its contents.

There is no bonanza. No motherload. Not even a speck of gold in the bottom of the pan. Before the Bill was published, the Scottish Government had serious concerns that it would prove a “small nuclear bomb for devolution”. Today’s verdict? Kaboom.

If it isn’t reserved, its devolved. This, in a nutshell, is the working assumption undergirding the Scotland Act.

In shaping devolution, Westminster decided to fence off war-making and nuclear weapons, diplomacy and currency and social security.

But if an issue isn’t on the list of reservations? Then Holyrood can decide. MSPs are a little more constrained under the Scotland Act.

Their decisions must also respect the fundamental rights protected by the European Convention – and EU law. And some areas of devolved regulation rely extensively on EU rules.

While Britain remained part of the Union – this structure ticked along perfectly well, but ignite the Brexit fuse, and the logic of the Scotland Act goes up in a puff of smoke. Many critical areas for May’s government – most prominently agriculture and fisheries – are not reserved to Westminster, but are currently extensively regulated under European frameworks.

Knock out those European frameworks, and the Scottish Government and Parliament would accrue dramatic expansions in their devolved powers. May seems – from her perspective, understandably – disinclined to leave the regulation of fisheries in Scottish waters to chance – or to the Scottish Government or Parliament.

So she hasn’t. The Prime Minister’s Brexit Bill leaves Edinburgh, Belfast and Cardiff’s parliaments all trussed up by the EU rules her government and parliament are empowered to sweep away. Modestly, in its characteristically strong and stable fashion, the UK Government has effectively appointed itself the all-powerful arbiter of everything. As one observer noted yesterday, “the irony of leveraging more reserved powers from the outrage of Brussels control” is bleakly comic.

This is a new controversy, but an auld sang. Yesterday’s Bill is yet another a monument to the old Whitehall mania for central control, punctuated occasionally by bleak charades of “consultation” and “negotiation” taking place, with comfortable irrelevance, against a backdrop of formal subordination.

David Davis describes these proposals as “a transitional arrangement while decisions are taken on where common policy approaches are or are not needed”. Note the passive voice, and take care to read between the lines. We can expect to hear a good deal more about “common policy approaches” in coming months from the UK Government, common sense, single UK market regulation – whatever the Scotland, Wales or Northern Ireland Acts might once have said.