In a rather desperate attempt to demonstrate that he's taking "tough" action on immigration, David Cameron has rushed forward a ban on migrants claiming out-of-work benefits for three months after their arrival to 1 January, the date when the transitional controls on Romanians and and Bulgarians expire.

He said:

The hard-working British public are rightly concerned that migrants do not come here to exploit our public services and our benefits system. As part of our long-term plan for the economy, we are taking direct action to fix the welfare and immigration systems so we end the 'something for nothing culture' and deliver for people who play by the rules. Accelerating the start of these new restrictions will make the UK a less attractive place for EU migrants who want to come here and try to live off the state. I want to send the clear message that while Britain is very much open for business, we will not welcome people who don’t want to contribute.

Based on these words, voters might reasonably assume that "benefit tourism" is one of the biggest problems facing the UK. But, of course, the reverse is the case. As a recent EU study noted, "the majority of mobile EU citizens move to another Member State to work" and benefit tourism is neither "widespread nor systematic". The DWP's own research found that those born abroad were significantly less likely to claim benefits than UK nationals. Of the 5.5 million people claiming working age benefits in February 2011, just 371,000 (6.4 per cent) were foreign nationals when they first arrived in the UK. That means only 6.6 per cent of those born abroad were receiving benefits, compared to 16.6 per cent of UK nationals.

But while blogs like this one and economists like Jonathan Portes repeatedly make this point, don't expect any of the main parties to do so. Labour's response to Cameron's announcement can be summed up as "it was our idea first!" Here's Yvette Cooper's statement:

Labour called for these benefit restrictions nine months ago. Yet David Cameron has left it until the very last minute to squeeze this change in. Why is the Government leaving everything until the last minute and operating in such a chaotic way? Three weeks ago Theresa May told Parliament she couldn't restrict benefits in time, now the Prime Minister says they can. They wouldn’t be on the run from angry Conservative backbenchers if they’d listened to us nine months ago. But while Cooper might be wrong to perpetuate the myth of benefit tourism, she is certainly right to note that Cameron is "on the run" from his recalcitrant MPs. Nearly 80 Tory backbenchers (almost enough to deprive the coalition of its majority) have signed an amendment ordering the government to break EU law and extend the labour market restrictions on Romanians and Bulgarians for a further five years (with a Commons vote to be held next month). Many of them will nod in agreement with Nigel Farage when he declares: "Smoke and mirror policy today by the Govt over Bulgarian & Romanian migrants, all to try shoot UKIP's fox. Without actually saying that."

In offering "tough" new measures on immigration, Cameron is seeking to appease the unappeasable. Why, his MPs and others will ask, should migrants only be barred from claiming benefits for three months? And if the PM can rewrite the rules to stop newcomers receiving welfare, why he can't he rewrite them to stop them taking jobs? (Many on the right appear to simultaneously believe that immigrants come to sponge off the state and that they're taking 'all the jobs'.) As Tory rebel David Ruffley said in response: "It's not enough to choke off any abuse of benefits because many want to come here to work.

"The minimum wage in Romania is £1 and, for perfectly rational economic reasons, they want to come here to work for £6 an hour. We were told 13,000 Poles were coming under the Labour government and it turned out to be 500,000, putting pressure on public services."

Rather than challenging those who believe that immigration is always and everywhere an ill, Cameron is reinforcing the view that we should do all we can to deter foreigners from coming to these shores. Again, as any economist will tell you, the reverse is true. There is no evidence that migrants take jobs that would have otherwise gone to domestic workers (studies suggest that immigration increases labour demand as well as supply), or that they depress average wages. But there is much evidence that they are net contributors to the economy, paying far more in taxes than they receive in benefits and services. An OECD report last month, for instance, found that they make a net contribution of 1.02 per cent of GDP or £16.3bn to the UK, since they are younger and more economically active than the population in general.

It's for these reasons that, as the Office for Budget Responsibility has shown, we will need more, not fewer immigrants, if we are to cope with the challenge of an ageing population and the resultant increase in the national debt. Should Britain maintain net migration of around 140,000 a year (a level significantly higher than the government's target of 'tens of thousands'), debt will rise to 99 per cent of GDP by 2062-63. But should it reduce net migration to zero, debt will surge to 174 per cent. As the OBR concluded, "[There is] clear evidence that, since migrants tend to be more concentrated in the working-age group relatively to the rest of the population, immigration has a positive effect on the public sector’s debt…higher levels of net inward migration are projected to reduce public sector net debt as a share of GDP over the long term relative to the levels it would otherwise reach."

One might expect a fiscal conservative like Cameron to act on such advice but, as so often in recent times, the PM is determined to put politics before policy. The irony is that, by allowing UKIP to claim yet another political victory, he isn't even succeeding in these debased terms.