Ever longer grows the list of potential disasters that lie ahead for Britain, thanks to Theresa May’s decision that, on leaving the EU, we should also leave its single market. But rumbling away now in the background is one that dwarfs them all, because it will affect every one of the 170,000 UK businesses that trade with the rest of the EU, and much else besides.

We are, of course, wearily familiar with the argument that, because the rest of the EU sells more to us than we do to them, they will happily concede us that “one-off deal” Mrs May wants, allowing us to continue trading much as we do now. But this relies on a complete failure to grasp the real nature of the regulatory system that is the essence of the single market, and what would be facing us if we leave it to become what it calls a “third country”.

We would be excluded from that fully computerised system which for 25 years has…