A good rule in international relations is: never issue a threat unless you are prepared to carry it out. For if you do, you only reveal to everyone how weak your negotiating hand actually is. That’s been the problem with Theresa May’s position on European citizens living in Britain ever since she first articulated it as Home Secretary in the days after the Brexit referendum. Last night in Brussels, she again said that they can remain here, but only if the same rights are extended to Britons living in European countries. Once again, this morning, millions of families wake up among us not knowing whether they will be allowed to remain in the country where they have chosen to make their lives. There has to be a very strong reason of national interest even to contemplate causing such upset. There isn’t.

It seems very unlikely Spain would kick out our expats on the Costa del Sol, or that France would depopulate the Dordogne. But even if they did, no one really believes that Britain would deport the three million European citizens who live and work here. Why not? Because it would be a massive act of self-harm. As Mrs May said in February: “EU citizens living in the UK make a vital contribution to our economy and society and without them we would be poorer and our public services weaker.”

Threatening to make yourself poorer and your NHS weaker isn’t very credible. Nor does anyone believe that, even if she wanted to, Mrs May has the numbers to carry out her threat. She has barely persuaded her Conservative colleagues to back her plan. Last June, in the days immediately after the referendum, David Cameron wanted to reassure EU citizens they would be allowed to stay. All his Cabinet agreed with that unilateral offer, except his Home Secretary, Mrs May, who insisted on blocking it. A vote in the Commons earlier this year was only carried with a nod and a wink to Tory MPs behind the scenes that she didn’t really mean it. Since then, the Government has lost its majority and it seems likely that an opposition motion to grant EU citizens the right to remain unilaterally could be carried.

So why prolong the uncertainty for these families? The Government should announce unilaterally that any Europeans who are living here will be able to remain here, work here, pay taxes here and use the public services they help pay for. You wouldn’t need any complicated reciprocal agreement with Europe, or provide any role for the European Court of Justice. It would be, as we said before, an act of national self-interest dressed up as a gesture of international generosity. It would almost certainly force the hand of European governments to offer the same to Britons. Morally right, economically sound and diplomatically smart: such opportunities don’t come along very often in life, and we should grab them.

Toxic air watch

Today, on page two of the paper, we publish for the first time London Air Watch — our guide to levels of air pollution in the capital, compiled independently by the weather experts MeteoGroup. It will become a new daily feature in the paper, so that our readers know whether or not they and their families are being exposed to illegal levels of toxic air.

The purpose is not just to inform the public but also to arm them with the facts to demand a response to the problem of pollution from local and national government. Our long-running campaign for clean air — particularly in respect of particulate emissions from diesel cars — has seen politicians from both Left and Right move from a position of indifference to concern. Now concern must become concerted action. Lives depend on it.