The posters warning illegal immigrants to “go home or face arrest” were bad enough, striking a bullying note more reminiscent of a British National Party rally than a responsible government campaign.

Now, though, evidence of immigration officials conducting spot checks at, for example, London Underground stations – perhaps targeting non-whites, in particular – is more disturbing still.

The Home Office defence of such heavy-handed, if not illegal, tactics is far from convincing. Upholding the law is one thing; stopping people at random and demanding their papers is quite another, as is using Twitter to trumpet tallies of arrests.

Immigration is one of the most complex and inflammatory areas of public policy. As such, it should be kept as far away from politics as possible. Instead, the Conservative-led Coalition – unimpeded, it would seem, by the feeble protests of its junior partner – is playing ever more stridently to the gallery, currying favour with the most narrow-minded and ill-informed of the electorate, and shamelessly following public opinion when it should be leading it.

Nor is the problem simply one of tone. The substance of the Government’s policies is equally wrong-headed. First, the notion of a cap on net migration levels – that is, the balance between arrivals and departures – is nonsensical, focusing on an arbitrary annual figure rather than on what the Government is trying to achieve (which remains unclear). Such an approach also assumes, with frightening oversimplification, that an Indian software developer, a Polish plumber and a British pensioner retiring to the Costa del Sol are interchangeable.

There is also a problem with the numbers upon which the tottering edifice rests. Given the sensitivity of the topic, official immigration statistics are almost comically woolly, the numbers of entries extrapolated from a tiny sample and the number of exits not counted at all.  Not only is the target the wrong one, then, but our means of establishing whether we have hit it are suspect.

Finally, there is the blunt reality of what a crimp on immigration means. The majority of people who come to Britain are from other parts of Europe and are fully within their rights to do so. To make up the numbers, the Government must therefore squeeze extra hard elsewhere. That means fewer visas for overseas students and skilled workers.

Never mind that our universities are a successful export industry (supposedly a government priority). Never mind that businesses are crying out for highly trained staff, and that the NHS relies on staff from overseas. Never mind, either, that migrants claim far fewer benefits than indigenous Brits, or that our ageing population calls for more newcomers, not fewer. The dominant rhetoric is, nonetheless, of a country swamped by foreigners either looking for handouts or deviously undercutting the locals.

Instead of confronting the quagmire of fear, loathing and misinformation, the Government is playing along with it. Bogus asylum-seekers (of whom there are few) are conflated with health tourists (again, very few) and with Polish plumbers (whose services are beneficial) to create a universal bugaboo upon whom all the ills of society can be blamed. Thus, Ukip-waverers are courted, very real policy failures – notably that British youngsters are so much less work-ready than their foreign counterparts – are dodged, and the debate is conceded to the ignorant and the downright bigoted.

Britain will be the loser, economically, culturally and socially – and all because David Cameron is spooked by Nigel Farage.