By Alastair MacIver

THE term “UK single market” has bubbled up in the wake of the Brexit referendum.

It peppers parliamentary motions and speeches by the Scottish Secretary. Scots are repeatedly invited to make a choice between the European single market and its apparent British analogue the “UK single market”.

This description of the British domestic marketplace is new, self-evidently purloined from the language of European integration and does the rhetorical work of presenting the UK as a microcosmic EU, with all of the same goodies in terms of unimpeded trade. This characterisation should be approached with extreme caution.

Indeed, no such “UK single market” currently exists and, in the event of hard Brexit, establishing an effective and lasting one would entail a large expansion of the reserved powers vested in Westminster.

This would see Holyrood lose some of its current powers and miss out on those “repatriated” from Brussels.

Why can we expect Westminster’s authority to expand in this way? In essence, if Holyrood and Westminster are allowed to regulate goods and services separately, obstacles to trade would inevitably be presented if a good or service had to follow different rules on each side of the Tweed.

For example, trade would be impeded if a Carlisle based hairdresser had to acquire a licence to coif in Gretna Green, or if a Scots haggis had to be relabelled to be sold in Berwick. These kinds of obstacles are the so-called “non-tariff barriers” that an excessive focus on the possibility of a hard border (with the attendant customs duties and checks) seems to forget.

In order to forestall divergences in how goods or services are regulated in different parts of the UK – that is, to create the real “UK single market” we keep hearing about – Westminster would need a general reserved power to lay down common trading rules. Such a power would be comparable to the one most central authorities enjoy in federal states, including, most notably, the “commence clause” in the United States Constitution.

The authority to enforce the rules would likewise need to be centralised. Mere technical points, you might think, but consider one current divergence in how goods are regulated in Scotland vis a vis the rest of the UK: minimum alcohol prices.

We know that EU rules potentially prohibited the Scottish Government’s flagship policy, but would the “UK single market”? In any event, Holyrood would clearly not be the recipient of key powers returning from Brussels, if Westminster’s authority to implement EU rules on trade in goods became the far wider power to make UK replacement rules.

While the UK is in the EU, the British national market is but a part of the European single market - an area in which free movement of goods, services, persons and capital is assured. We underestimate at our peril the extent to which the devolution settlement of 1998 was premised on a European platform of rules followed by Westminster and Holyrood alike.

Brexit upsets that assumption. To date, the work of ensuring unhindered trade between a devolved Scotland and the rest of the UK has been carried out by the EU. If the UK is to leave the European single market and a UK version is to be erected in its place, Holyrood looks set to lose out. To put it another way: while the cake of potential powers will be larger, Westminster looks primed to seize the biggest slice.

Alastair MacIver is a Lecturer in Law at Tilburg University, Netherlands