It remains a mystery why the Prime Minister has recommitted her party to reduce net migration to the tens of thousands a year. She didn’t need to make this politically rash and economically illiterate move. She was not the author of the pledge; David Cameron made it in opposition.

She knows better than almost anyone that net migration — the number of people arriving, minus the number leaving — is not in the gift of government, subject as it is to the vagaries of the world economy. Moreover, this target has a perverse incentive, in that the more people you persuade to emigrate from the UK, the more likely you are to hit it.

So you would assume that Mrs May would jump at the chance to bury the pledge. That’s what her Cabinet assumed; none of its senior members supports the pledge in private and all would be glad to see the back of something that has caused the Conservative Party such public grief.

But no. Mrs May has kept digging. So, how to achieve this reduction? Brexiteers suggested leaving the EU would keep foreigners out. They never explained that of the net migration figure last year of 273,000, 164,000 people came from outside the EU. As David Willetts pointed out here yesterday, we already have levers to reduce their numbers, but have sensibly not pulled them.

To meet the pledge, the number of both EU and non-EU migrants will have to be reduced by around two thirds. How is that to be achieved? No one in government can identify the third we want and the two-thirds we don’t. Asked whether we want to stop bankers, builders, berry pickers or baristas coming from Europe, ministers are at a loss. The Business Secretary, Greg Clark, could not name a single sector that should have its supply of labour forcibly reduced. He knows that to do so would push up prices and hurt firms. Rohan Silva makes a similar point here.

Falling unemployment

Instead, ministers assert we need to train under-qualified British people to fill posts currently filled by over-qualified Europeans. But where is this army of workless Brits? The numbers claiming out-of-work benefits is at a 45-year low: today’s unemployment figures show a decrease of 53,000 between January and March, to 4.6 per cent. No one has the stomach to tackle the thorny issue of people who are capable of working yet claiming sickness benefits.

What about the many Europeans who attend our universities? European students are a vital source of income and academic excellence for our colleges, and one would presume that a “global Britain” would want to export education and influence. Nor have people been told that they won’t be able to marry a Danish boyfriend or French girlfriend and bring them to Britain. So who are the 100,000-odd Europeans we are supposed to be turning away? When the debate turns from the generality of reducing European migration to the specifics, the Government is floundering.

We have been here before. Over the past seven years, the Government has not been able to reduce significantly the numbers of non-Europeans coming here — though we could. The damage to the economy from seriously reducing work visas was judged too severe by an expert migration committee; the impact on community relations of further limiting family reunion visas was seen as unpalatable; and few thought we were taking in too many refugees. There are no other groups we can turn away.

Mrs May knows all this. She knows that a sensible immigration policy is driven by clear principles not arbitrary numbers. If one of those principles is no longer to be the freedom to move to work between Britain and Europe, we need to hear what its replacement will be. Recommitting to a failed immigration pledge, without knowing how to achieve it, is merely wishful thinking. She still wants to be a new broom. She should use the Tory manifesto tomorrow to sweep away this bad policy from the past.