Few of those who voted for Brexit, it is safe to say, did so for the purpose of extricating the UK from the customs union. The free movement of people, sovereignty, and weariness with austerity were all significant factors in the result - but the desire for Britain to strike its own trade deals was not.

Theresa May has nevertheless argued that remaining in the customs union would “betray the vote of the British people”. The Prime Minister may yet be forced to do just that.

MPs will soon vote on whether Britain should seek a customs union with the EU (a moment the Conservatives have consistently postponed for fear of losing); Brussels has warned that there is no alternative means of avoiding a hard border with Ireland. At a Press Gallery lunch earlier today, Home Secretary Amber Rudd refused to confirm that the UK would leave the customs union (“I’m not going to be drawn on that, we still have some cabinet discussions to have”).

Like May, other Tories have insisted that they have an unambiguous mandate for customs union withdrawal. Sajid Javid, the Communities Secretary, tweeted: “British people gave politicians clear instructions through EU referendum. Includes leaving the Customs Union, an intrinsic part of the EU.” Michael Gove, unsurprisingly, seconded Javid: “Sajid is right. The referendum vote was clear - we need to take back control of trade - that means leaving the protectionist customs union.”

Yet one searches in vain for such language during the referendum campaign. Among Vote Leave’s 53 “key speeches, interviews and op-eds”, encompassing tens of thousands of words, there is just one reference to the “customs union”

In no reasonable sense, then, can the British people be said to have given their consent to leaving the customs union. The referendum result provided a mandate for leaving the EU and, arguably, for ending free movement - but it did not commit the UK to a “hard Brexit”.

By the time of the 2017 general election, the Conservatives had stated repeatedly that the UK would leave the customs union. But as is well-documented, they emerged with no majority and no mandate. Yet May, mindful of her party’s regidicial Brexiteers, has since acted as if “nothing has changed”.

The same voters who the Prime Minister claimed a customs union would “betray” have been shown by polls to favour one. On this, the public’s intuitions are correct: the costs of leaving the customs union outweigh the benefits. Indeed, the government's own analysis suggests that the UK would lose between 2 per cent and 8 per cent of GDP over 15 years from a “hard Brexit” (withdrawal from the single market and the customs union), while new trade deals with the US and others would add no more than 0.6 per cent.

For the Brexiteers, exit from the customs union, would herald the birth of a freewheeling, buccaneering “global Britain”. Rather than being shackled to Brussels, the UK would be liberated to strike valuable trade deals with China, India and “the Anglosphere” (the US, Canada, Australia and New Zealand).

Yet there is no majority in parliament or the country for the policies this would entail: low taxes, low regulation and cheap imports. Most Leave voters crave more state intervention, not less. Mindful of this, the Brexiteers are careful to speak in generalities, rather than particulars.

A free trading, “global Britain” would devastate the UK’s agriculture sector. The mere prospect of chlorine-washed chicken from the US was enough to divide the cabinet last year. Without the negotiating heft of 27 other member states, the UK would struggle to achieve beneficial trade deals. The US would demand greater access to agriculture and the NHS. India and China would demand more visas (a problem for immigration-averse voters).

The weakness of their economic case explains why the Brexiteers place such emphasis on the supposed “mandate” for customs union withdrawal. But as so often, this boast has no basis in reality.