In an otherwise mixed bag of claims, the White Paper Scotland’s Future published by the Scottish Government in 2013 got one thing right.

It pointed to a”new threat”, the “growing possibility” that Scotland would be taken out of the EU against its wishes. That prediction, a sort of mini version of Project Fear, didn’t cut much ice at the time. The claim by Better Together that voting to stay in the UK was the best way for Scotland to stay in the EU got rather more traction. The authors of the White Paper might congratulate themselves on their prescience, but they perhaps didn’t appreciate just what a complicating factor Brexit would be for their goal of independence.

For while Brexit might make a vote for independence more likely, it makes the delivery of an independent Scotland very much more complicated. More likely because Brexit has had a forcing effect on opinion, pushing “Remain” voters towards the nationalist camp and probably accounting for the modest, but sustained, uptick in support for independence since the 2014 vote. As the drag of Brexit on the UK’s economic growth becomes more evident, the Brexit effect on support for independence could be enduring.

More complicated because Brexit confronts an independent Scotland with a hard choice, between unfettered access to an internal market on these islands or membership of the EU and its single market and customs union. This matters a lot. With 60 per cent of Scottish exports destined for the rest of the UK, any check on the free flow of these goods and services will have a potentially severe economic impact. As Brexit reminds us daily, distance still matters in trade. In unhindered markets, countries will naturally trade more with their immediate neighbours. New trade barriers with Scotland’s main and closest market will reduce long-term prosperity.

Before Brexit, this was simply not an issue. Had an independent Scotland joined the EU then, trade with the rest of the UK would have been folded into the wider EU single market. There would have been no tariffs and, more importantly, no non-tariff barriers, the regulatory, security and other checks that Scottish businesses are now finding so burdensome as they trade with the EU post-Brexit.

With the UK outside of the EU, this simplicity is gone. It can only return if the UK comes back to the EU fold; a vanishingly distant prospect on the sort of timetable now set out by the Scottish Government for another independence referendum.

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This, then, is the nature of the choice that would confront an independent Scotland. In a series of interlocking negotiations, it would face two of critical importance to Scotland’s trading relationships. The first, with the Government of “remainder” UK, would have to cover the rules governing trade between the two sovereign states. This is unavoidable; there will inevitably be a gap between Scotland leaving the UK and Scotland joining the EU.

What would nature of that trading relationship be? Whisper it softly, but the overwhelming economic interests of both parties, in particular the nascent Scottish state, would be to cause as little disruption as possible to existing trading patterns. An independent Scotland would face enough economic challenges without adding a disjuncture in the flow of trade with its main market. The outcome would matter less to “remainder” UK; as with Brexit, the larger party is comparatively less impacted by new trade barriers. But there would be sufficient specific business interests to prompt the Government of “remainder” UK to seek to protect trade, at least in the short term.

There is no guarantee, of course, of a mutually beneficial outcome. The British state has shown itself quite obtuse enough with is reductive deal with the EU to shoot itself in its economic foot. And who knows what political pressures would rock the incipient government of an independent Scotland. But there would be a chance that the two sides would agree a functioning trade relationship.

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As a new third country, an independent Scotland would have at the same time to negotiate its own relationship with the EU, including on trade, pending negotiations on accession. That would throw up its own eddies of complexity, particularly if UK standards have by that time diverged significantly from those of the EU; there would be limited options other than to maintain something akin to the current UK/EU arrangements.

The second crucial set of negotiations would be on accession to the EU. There is no reason to assume that these would not be successful. But then the people of Scotland would be confronted with the reality of another hard choice: reject membership of the EU to stay in a single market with remainder UK or throw in their lot with the EU.

There is no way round this. Joining the EU single market would mean a trade border with remainder UK. That would mean all the trade-diverting impacts of customs and regulatory checks, rules of origin requirements and, potentially, tariffs. Moreover, this would require some sort of physical infrastructure; there is barely a trade border in the world that does without. Welcome to the check point on Carter Bar. Would not technology provide a solution? That was the fantasy that beguiled all manner of Brexiteers and foundered on trade realities. What about a sort of Northern Ireland Protocol for Scotland? This would be a tough ask for an acceding member state; why would the EU complicate further the protection of the integrity of the single market? Besides, as we know from the implementation of the Protocol itself, it does not do away with the need for border checks.

An independent Scotland choosing to join the EU might not be as momentous a moment as leaving the UK. But nonetheless it would be a decision which would compound the economic fracture with Scotland’s main market. Many will cavil at the way Brexit has foisted this dilemma on Scotland; but the hard fact is that this choice will have to be made if Scotland becomes an independent nation again.

Philip Rycroft was Permanent Secretary of the Department for Exiting the EU between 2017 and 2019.