If the people of Scotland vote to leave the United Kingdom this week, customs checks along the new border are practically unavoidable. Alex Salmond claims that Scotland will continue to be part of the EU. He's bluffing, and without membership of Europe's Customs Union, his newly independent country will no longer be able to export goods to England tariff-free: anything crossing the border must be examined and taxed, and a cut sent off to Brussels.

It's perfectly possible for Scotland to rejoin the EU after 2016, and the difficulty of doing so is being exaggerated by unionists, but automatic membership is legally impossible. The members of the European Union are states, not peoples or territories, and to gain membership a state must be approved by the governments of all the other EU member states.

Except in France.

Under Article 88-5 of the French constitution, the French Government no longer has the power to approve new EU member states by itself. The political elite there is so distrusted that new states must be approved by the people in a referendum, or by a supermajority in Parliament. Has Mr Salmond made a secret deal with the French people, or perhaps with the rightwingers and nationalists of the French opposition, or is he just winging it again? The French are not going to ignore their own constitution to help the SNP, as letting the French political class admit new countries to the EU means letting Turkey into the EU, and that is about as popular in France as cutting agricultural subsidies, so somehow the politicians or the people need to be bargained with.

Even if Salmond said he hadn't made a secret deal with any foreign rightwinger other than Rupert Murdoch, we should not believe him: his administration spent thousands of pounds of taxpayer's money trying to prevent the disclosure of legal advice on Scotland's EU membership when in fact this advice didn't exist in the first place. Would you believe non-existent advice from this man?

For an indpendent Scotland to rejoin the EU, it needs to conclude a treaty with the existing member states, including the rump UK. This can't be done while Scotland is still part of the UK (particularly from the perspective of the French constitutional requirements). This means months or years of disrupted cross-border trade and customs checks along the Tweed, at Euston Station and so on while the other countries sign up.

Much has been made of the potential attitude of governments in Spain, Cyprus, Greece and other places with sensitivities about secessionism. Particularly troublesome are countries such as Spain and Cyprus which have territorial claims against the UK in Gibraltar and Akrotiri. Foreign politicians can demand that the UK abandon naval and military bases in the Mediterranean as the price for restoring customs-free trade in Great Britain. This is not a situation David Cameron should have allowed to come into existence. The Scottish Government has spent taxpayer's money brushing off Freedom of Information requests asking what discussions they have had with Spain and Cyprus over this issue. Maybe those discussions didn't happen either!

Scotland may well be better off outside the EU; after all, small non-EU states like Norway and New Zealand do fine, but pretending that the country won't spend a day outside the EU is insulting to Scottish voters and the nation as a whole.