BREXIT poses a real challenge to devolution as we know it. There are not many things which Theresa May and Nicola Sturgeon agree on. But this is one of them.

The Prime Minister and First Minister this week both raised the prospect of a profound rewriting of Scotland’s place in the Union after the UK leaves the European Union. For Mrs May, this looks like an opportunity. For Ms Sturgeon, it is a threat. So battle lines are drawn.

Mrs May yesterday told the Scottish Conservatives the current devolution settlements, almost two decades old, were of a bygone age. They were, she said, “designed in 1998, without any thought of a potential Brexit”.

Earlier in the week Ms Sturgeon said Tories would exploit Brexit to “strip” Holyrood of powers, not least over fishing and farming.

Both views are perfectly logical and both stem from a simple reality of Brexit that, alarmingly, was overlooked during last year’s EU referendum.

That simple reality? That creating a new British single market – to replace the larger EU one – meant imposing a single set of British set of rules and regulations. And that, in turn, left little room for Scotland to pursue its own path on a whole range of issues, especially on those currently settled in Brussels.

It took a Scot working at a Dutch university to spell this reality out. As politicians in both Westminster and Holyrood speculated about Brussels powers being repatriated to Edinburgh, EU law expert Alastair McIver warned the opposite was more likely. Writing in this newspaper’s Beyond Brexit series last year, Mr McIver predicted Westminster would take the lion’s share of repatriated powers to ensure there were no non-tariff barriers between England and Scotland.

This challenges the very basis of the current settlement: that what is not reserved is devolved. The UK may feel the need for a US-style “commerce clause” on whole areas of Scottish public life, such as health or transport policy so that it can enter in to global trade deals.

Mrs May’s supporters will rightly stress the dangers of any barrier to trade between Scotland and the rest of the UK. Ms Sturgeon will, however, equally rightly point out there are also dangers in narrowing this country’s interest to the newly coined UK single market.

Giving the UK control over agriculture – such as rules for the transportation of livestock or meat safety – would ensure a UK single market. But it would also generate a whole new series of non-tariff barriers for Scotland’s huge food and drink exports to the EU.

Completely free and unfettered trade with England and the rest of the EU are both essential for Scotland. They are also – after Brexit – both impossible. This means Scots have to make tough choices, and that includes on what Holyrood administers.

Mrs May’s centralist rhetoric may be buoying a new brand of hardline Unionist. But she takes a big risk if she thinks Ms Sturgeon is bluffing on a second independence referendum The Prime Minister describes the old Whitehall policy as “Devolve and forget”. Now it appears to be “devolve and meddle”. Ms Sturgeon knows Scots voters may not put up with that.