Christopher Howarth is a senior researcher working in the House of Commons. Prior to this he worked for Open Europe, as a Conservative Foreign Affairs Adviser and senior researcher to a Shadow Europe Minister.

Nicola Sturgeon has demanded an immediate referendum on Scottish independence. No surprises – she is a Scottish separatist after all. But don’t be fooled: the shrill and insistent tone of this new demand is not the product of a confident, separatist movement, but the result of a growing an irreversible weakness in the intellectual case for independence.

For not only do polls show that Scots do not seek a new poll, and would not vote for separation if there were one,  but the alternative that  the SNP is offering – immediate EU membership – does not exist and, even if it did, would not suit Scotland’s economic and political interests.

The high oil price, the ‘arc of prosperity’ and an EU that in the event of independence would straddle both the rest of the UK and Scotland all seemed to make Scottish separatism possible. However, the ark has sunk in a world awash with oil –  leaving the EU as the remaining piece of geopolitical glue to hold together the SNP’s prospectus.

For this reason, Brexit is the SNP’s kryptonite – it blows away their last fantasy. Sturgeon is not happy. She fumed at the Prime Minister’s decision:  “If she’s talking in the spring of 2019, a bit later perhaps than I was suggesting, there may be some room for discussion around that. But it seems to me to be just fundamentally unfair for a UK government, with Brexit having sunk the ship, trying to puncture Scotland’s lifeboat as well. I don’t think that is reasonable because by that point Scotland has been taken out of the EU – two years have elapsed.”

Putting aside the rather naive belief that the Prime Minister of the United Kingdom should be “reasonable” to a politician that is trying to destroy her country: Sturgeon’s answer acknowledges her argument’s fundamental weakness. Once the UK has left the EU, by March 29 2019 at the latest, the First Minister’s chances of leading Scotland to separation are over. She is fresh out of options: post 2019, she cannot hide the fact that independence means independence – hard independence, to coin a term.

So hard, indeed, that Scotland would be out of the EU, the EEA and, presumably, NATO given the SNP’s stance on nuclear weapons. Such independence would leave Scotland free to seek a free trade agreement, a customs union or even a currency union with the rest of the UK and could even work…but is not a vision that the SNP could sell in Scotland.

Non Option 1 for Scotland: Immediate EU membership

To join the EU you need to be an EU candidate; to be a candidate you need to be an independent state. Scotland would therefore need to complete its divorce from the UK and then apply for EU membership. It would then need to meet the Copenhagen criteria, show that it had built the administrative capability to live up to its EU commitments, pass 35 accession chapters, gain unanimous approval of all other EU states – and accept all of the EU treaties.

The UK has a number of treaty opt-outs and negotiated positions that Scotland would not automatically take advantage of. Scotland would not have a budget rebate as the UK does. It would have to adopt the Euro – which means setting up its own currency and joining the Exchange Rate Mechanism as a forerunner. Only Denmark and the UK have legal opt outs from the Euro. If Scotland decided to use the UK Pound unilaterally, it could not even pretend that it intended to join the Euro.   Scotland would also need a Schengen opt-out in order to continue in the Common Travel Area with the UK and Ireland, and thus maintain passport free travel. That might be possible, but extracting every concession takes time.

In any event, EU membership would be a particularly bad deal for Scotland. Independence would leave the rest of the UK as a destination for 63 per cent of Scots exports behind a customs barrier – and, while it might be possible to reduce the inconvenience by making this border frictionless, it would leave Scotland left out of important UK trade deals. While the UK can prioritise Scots exports such as whisky, the EU has failed to do so. Scotland would also have to re-join the Common Fisheries Policy.

Non Option 2 for Scotland: EEA membership

With immediate EU membership off the table, the SNP have flirted with the idea of joining the EEA along with Iceland, Norway and Lichtenstein. This would leave Scotland with good access to the EU market, but without any influence over the rules and in a different regulatory zone to the rUK. Perhaps the SNP would be happy with this? That is however a moot point – since EEA membership is off the table as well.

To join the EEA, the agreement states in Article 128 that: “any European State becoming a member of the Community shall, and the Swiss Confederation or any European State becoming a member of EFTA may, apply to become a party to this Agreement. It shall address its application to the EEA Council.”

Scotland would therefore need to apply as a state after independence, and after joining EFTA as a member. And to join EFTA, Article 56 of the EFTA conventions states: “any state may accede to this Convention, provided that the Council decides to approve its accession, on such terms and conditions as may be set out in that decision.”

In short Scotland, would first need to leave the EU or EEA in order to seek a way back in.

Non Option 3 for Scotland: EEA membership while in the UK

This option is not really an option as it has been ruled out by the UK Government, and for good measure by the Norwegian and Icelandic Governments. Like the options above, it suffers from the key fault, in that you have to be an independent state to be a member. It would also create a regulatory frontier within the UK – something that would be impossible to police and cause legal chaos.

All this leaves the SNP is difficult territory. The UK Brexit referendum campaign saw a momentary and curious alignment of Conservative remainer Unionists and Scottish nationalists, both convinced that Brexit would cause the end of the UK. Fortunately, they were a hundred per cent wrong.

We could have done more to strengthen the United Kingdom pre Brexit and should do more afterwards, but don’t be fooled: Brexit is no friend of the SNP.