Lord Green is President of MigrationWatch UK and a cross-bench peer.

It seems that we have been bounced into “threatening” to permit three million Hong Kong residents to settle in Britain whenever they so choose.

Out of the blue, the Government recently announced that, if China imposes their national security law on Hong Kong, they are prepared to offer residence in the UK to about half the population of our former colony. Amazingly, this fundamental change of policy seems to have emerged from some remarks by Dominic Raab to reporters.

At this point, a reminder of the history is needed. These citizens of Hong Kong possess, or will be able to apply for, British National (Overseas) Passports, which are travel documents that the Government granted only to Hong Kong, and only to those born before the 1997 handover to China. They do not, at present, grant a right to live in the UK but they do allow holders to visit for up to six months without first applying for a visa.

However, the Foreign Secretary has now told Parliament that, should the Chinese government continue “down its current path,” he will permit the document to be used for a visit of twelve months, which would be renewable and would permit applications for work and study.

This would become a pathway to full British citizenship after five years. It would also, of course, lead to access to the welfare state, health and education (and the right to vote in general elections).

A few days later, the Prime Minister leapt in with his characteristic enthusiasm. He warned China that, if they imposed such a law. He would offer BNO passport holders the prospect of British citizenship. This he wrote would amount to “one of the biggest changes in our visa system in history”, a step that the Government would willingly take.

So what could this possibly achieve? It is impossible to say what the take up would be. A survey reported by a Hong Kong newspaper in June found that 37 per cent were thinking of emigrating, up from 24 per cent in March. Of those planning to leave 90 per cent said that they were influenced by “the present situation.”

The Government claim that the numbers would be small. If so, how does this amount to a threat to China? They might well be delighted to see some of their key opponents leave.

If numbers were high, the Government would face major problems settling large numbers of new arrivals, especially if we were simultaneously facing unemployment of several million. Indeed, the rate of outflow could become a weapon in Chinese hands and, if they were nasty enough, those four million without BNO passports (who already have visa free entry to the UK) would have an arguable claim for asylum in Britain. Clearly, this huge change in our visa system is a massive hostage to fortune.

The potential scale of the consequences is, indeed, huge. There are about 300,000 holders of these British National (Overseas) passports, and nearly three million more whose passports have expired, but who have the legal right to renew them.

Nobody knows, of course, what proportion of these Hong Kong citizens would come to Britain. Some might prefer Taiwan. Others might prefer North America or Australia. All those countries would take some but we are, in effect, offering an open door to all three million BNO passport holders. This is 25 times the number of skilled work permits issued each year to non EU citizens and their dependants.

And all this is occurring as unemployment in Britain shoots up. Two million are already claiming unemployment benefit and several million more might be doing so before the year is out. British workers would regard such extra competition as extremely ill-timed, to say the very least.

Then there is the question of payment for all the additional housing, medical facilities, school and infrastructure needed. Three million is equivalent to the 2019 populations of Birmingham, Leeds, Manchester and Edinburgh added together. Not all will come and many of those who do will contribute, of course, but nothing like enough to pay for the infrastructure that would be needed.

Some commentators point to the Asians who were expelled from Uganda in the early 1970s. Many have done well here but scale is important. They numbered some 30,000 (including the parents of our current Home Secretary). The number of Hong Kong Chinese with the right to come here will be one hundred times the number of Ugandan Asians who arrived in the 1970s, not to speak of those who do not possess a BNO passport but who might claim asylum.

In short, this is an extraordinarily ill-conceived policy which, apparently, the Government intend to bounce through Parliament. So far, they are being given an easy ride by the opposition parties. Lisa Nandy, the shadow Foreign Secretary, has described the Government’s policy as “an important first step in fulfilling our longstanding obligation to the people of Hong Kong”.

Nandy appears to have glossed over the fact that Labour was in office for 13 years immediately following the handover. She is only outflanked by the Liberal Democrat call for all 7.4 million Hong Kong citizens to be offered the right to life in the UK. It is hard to imagine a more ludicrous suggestion.

But it is the Conservatives who are in power and must take the decisions. All this comes on top of the Government’s post-Brexit policy, which could substantially increase immigration from around the world. It lowers the salary and qualification requirements for work permits. It abandons any requirement to advertise jobs in the UK before looking to recruit overseas and, crucially, it drops the concept of a cap on the number to be issued each year.

If they now grant access to a large number of Hong Kongers they will have demonstrated beyond doubt that they have lost the plot on immigration and voters, especially in the North and East will draw their own conclusions. It is no exaggeration to say that the Government’s extraordinarily ill-considered policy on Hong Kong could cost them the next election.