After scepticism that David Cameron would even get close to his goal of reducing net migration to "tens of thousands" a year by the end of this parliament, the Conservatives have recently hailed their success in cutting it by a third since 2010. But today's figures from the ONS show that, after five successive quarters of decline, the numbers are now moving in the wrong direction.

In the year to December 2012, net migration (the difference between the number of emigrants and immigrants) stood at 176,000, up from 153,000 in the year to September 2012. This was due to a decline in emigration (which fell from 351,000 to 321,000), rather than an increase in immigration (which fell from 566,000 to 497,000), but since Cameron chose to adopt the net migration figure as his metric of success, the trend is politically problematic (if not economically problematic. As I've previously noted, migrants contribute far more in taxes than they receive in benefits and services and the OBR estimates that Britain needs net migration of around 140,000 a year to limit the increase in the national debt to 99 per cent of GDP.)

Worse for the Tories, it will become even harder for them to reach their target once the transitional controls on Romanian and Bulgarian expire on 1 January 2014. While there are likely to be far fewer new arrivals than UKIP and the right-wing press suggest, the Migration Matters Trust estimates that at least 20,000 will move to the UK. With Cameron powerless to restrict EU immigration, owing to the principle of the free movement of labour, the likelihood is that the total number will rise. Ahead of next summer's European elections, that will prove a political gift for Farage and co.