Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party and CEO of Brexit Analytica.

Why does the pound fall every time the Prime Minister talks about Brexit? Because it reminds the markets that two of her government’s aims: reducing immigration, and leaving the jurisdiction European Court of Justice, are incompatible with anything other than a hard Brexit. Leave aside the merits of each: what matters to business is that they both entail leaving the Single Market.

Though it might be theoretically possible to construct a free trade area in which goods, services and capital but not people move freely, it is not something that the European Union can concede. Attempts to carve up the Single Market go against the EU’s founding principles. When British pro-Europeans advance such schemes, they only betray how little they understood the European project, and give rise to the thought that it is better for them that we are out.

Leaving the jurisdiction of the ECJ is not only politically incompatible with staying in the Single Market: it’s conceptually impossible to reconcile the two. The Single Market is a single legal area and needs to be run by a single court. Though Norway is technically inside it without being under its jurisdiction, this is a fiction. Norway has formal freedom to depart from EU rules, granted on the understanding that it is never to be used. Leaving the ECJ’s jurisdiction only to put ourselves under the jurisdiction of another essentially identical body would be just the kind of prestidigitation Theresa May abhors.

Hard Brexiteers, of course see leaving as an end in itself. They think it will enable us to strike better trade deals than the one we currently have with the rest of Europe. Change Britain, which calculated these figures in Euros, argues that if we strike the right trade deals we could increase our trade with countries like Chile, South Korea, the United States by €23 billion a year (£20 billion), and that this would generate almost 400,000 jobs.

Assume, for the sake of argument, they are right. Using statistics from HMRC and the OECD, our trade with the EU is worth about £575 billion a year – almost 30 times as much. It follows from this methodology that the Single Market sustains more than 11 million jobs: more than 17,000 per parliamentary constituency, and about a third of all jobs in the UK. Of course, nobody would suggest that trade with the EU would fall to zero (even Guy Verhofstadt does not plan to impose trade sanctions on the UK): the important figure here is the factor of 30. EU trade would only have to fall by three per cent for the gains to be wiped out. It is not that there are no trading opportunities from Brexit, it is just that even according to Brexiteers they aren’t very large.

As an economic proposition it does not add up. Nor can savings from EU contributions make up the difference. If we were to add to this £20 billion a further £15 billion (£10 billion from EU contributions repatriated, spent at home, and postulating a generous multiplier of 1.5) we still only arrive at maximum annual benefits of £35 billion. Deregulation might help, don’t expect a Prime Minister who denounces the “cult of individualism” and plans to “step up” and “go further” to do much of it.

In any event, these trade benefits would be available if we negotiated an arrangement like Norway’s, outside the Customs Union, but inside the Single Market. We don’t even need to risk EU trade to get them!

Leaving it can’t be justified on economic terms, but is the unavoidable consequence of regaining sovereignty over immigration and legal interpretation. This may be what the British people said they wanted, but it isn’t what British business needs.

It is hard to overestimate how radical a shift in British economic policy this is. The currency markets are reminded of it every time the Prime Minister talks about Brexit, and they sell the pound in response. This has a paradoxical consequence: the foreign earnings of the British multinationals, which are measured in pounds, go up. Like in Newton’s law of motion, each fall in the pound produces opposite rise in the FTSE. It does not represent a sign of confidence in the British economy, but merely the belief that conditions in the rest of the world haven’t changed.

This is a shift for which British business, and the lobbying industry that failed abjectly to stop Brexit, is unprepared. They are used to a government that broadly shares their outlook and interests, not one indifferent to the collateral economic damage its own nationalist agenda will cause.