Many commentators predicted that immigration would lose political salience during 2011. Back in the spring, I explained here why I thought this unlikely. Economic downturns tend to heighten concerns about migrants competing for jobs, and exerting downward pressure on wages. Public spending cuts tend to sharpen debates around migrants adding to the pressure on public services, social housing in particular, and around migrants claiming benefits. Beyond the economic aspects, there is no sign of any decline in the sense that immigration is undermining or threatening our "way of life". And while neither Labour nor the Liberal Democrats are keen to talk about immigration, the Conservative leadership clearly believe it has a role to play in handling the right-wing media, as well as those on the right of their own party who are unhappy with life in coalition.

Immigration hasn't been all good news for the Conservatives. Last month, they had their first experience of a political crisis driven by operational failings at the border. At the same time the Office of National Statistics confirmed that net immigration was the highest on record. Three quarters of people were already doubting that the Conservatives would deliver their pledge to cut net immigration to tens of thousands, and their lead on the issue has fallen to 13 per cent, half of where it was last summer, and eroded further by the recent crisis.

Net immigration -- total immigration, including British citizens, minus total emigration -- was always a slightly quixotic choice for an overall political target. As the ONS explained, while immigration has been stable since 2004, net immigration has fluctuated with rises and falls in emigration, which is even harder to control. It is tempting to conclude that the Conservatives have given themselves an unnecessary political problem. But to understand why they did it, we need to go back to a couple of years before the election, when the pressure group MigrationWatch were exerting a lot of influence on the immigration debate, mainly through the right wing media, and also through their parliamentary wing Balanced Migration, fronted by Frank Field and Nicholas Soames. Their narrative relied heavily on the idea of Britain as a "crowded island", a clever tactic for reassuring moderates that there was "nothing racist about opposing immigration" -- it was simply a question of numbers. If crowding is the primary issue, then net immigration is indeed the logical target.

But this also explains why, as some on the right are making sympathetic noises about cutting ministers some slack on immigration, MigrationWatch are redoubling their campaign. When the ONS released the latest population projections, suggesting that the number of people living in the UK will rise from 62.3 million to the totemic 70 million figure within twenty years, MigrationWatch responded with an e-petition calling for the government "to take all necessary steps to get immigration down to a level that will stabilise our population as close to the present level as possible." Backed by the Mail and Sun, the country's two biggest selling dailies, the requisite 100,000 signatures were gathered within a week, and a parliamentary debate is expected in the New Year.

But rather than yet another debate over immigration (during which you can safely assume that many participants will use up a good share of their time complaining about the fact that they aren't allowed to talk about it), we should take the petition at face value, and actually try to have a debate about population, and ideally one which is informed by a deeper sense of the facts.

As I pointed out when the e-petition was launched, the latest UN figures rank the UK as the 39th most crowded out of 196 nations, and 140th in terms of population growth. Although many of the most densely populated are small islands or city states, Japan, South Korea, Belgium, and the Netherlands would all remain more densely populated than the UK even if we hit 90 million, never mind 70 million. MigrationWatch complained that I "carefully chose the UK rather than England" in making this comparison, but -- besides the obvious points that their own petition focuses on UK population figures that talking about this "crowded island" implies we are talking about Scotland and Wales as well as England, and that immigration policy is set at the UK level -- the real problem for MigrationWatch is that as soon as we move away from debating immigration and population at the UK level, why stop at England? Why restrict ourselves to a single conversation about a geographic entity whose regions and cities vary so markedly in these respects?

For example, northerners are among the most concerned about immigration, and also very concerned about population growth and crowding, despite having the slowest rate of population growth in England, with many towns and districts shrinking. The latest ONS figures show that the north-east has around 300 people per square kilometer, just above the UK average, and a third lower than the south-east. London is obviously the densest, at 5,000 people per sq km (though this is not particularly high either, by international standards or London's own history), and unsurprisingly, Londoners are among the most concerned about crowding. But despite this, and despite also having the largest number and proportion of migrants, attitudes to immigration in London are by far the least negative. A recent survey found 46 per cent of Londoners think immigration needs to be reduced, compared to 75 to 80 per cent nationally. (This is not just for the obvious reason that immigrants are themselves the most positive about immigration: the same survey showed that Londoners who identified themselves as "white British" were also significantly less likely to support cuts to immigration than "white British" respondents elsewhere in the country.)

London's population has been growing steadily since 1988 -- after shrinking for several decades in the middle of the last century -- and this year's London Plan forecasts that the city will grow by around 1.2 million in the next twenty years. On average, there is still a net outflow to the rest of the UK of around 10,000 a year, but this is far outweighed by two other trends: immigration from overseas, and birth rate. In 2009, around 150,000 immigrants arrived to live in London -- around a third of the UK total -- with a net immigration figure of around 30,000. London also had more than double its fair share of the country's natural population growth, of around 80,000.

Is there such a thing as an ideal level of population, either for the UK, or for the north, or for London? How far should government -- at national level, or in London's case, the Mayor's office -- try to intervene in that? If we believe that government should have a population policy, should it restrict itself to a "one-club" policy of managing immigration, or should it also intervene in family planning -- and what about the upward trend in life expectancy? Less controversially and probably more usefully, how we should plan to deal with whatever rises are expected? Finally, as we welcome the seven billionth member of the human race, how do these national or local concerns fit in with global questions around population growth, migration, urbanisation, resource scarcity, emissions, and so on?

These are some of the questions that will be discussed, in a London context, at a debate which IPPR is holding on December 12th as part of the London Policy Conference. The debate will be chaired by the New Statesman's own Mehdi Hasan, with expert contributions on migration, demography, urban planning, and economics, as well as the Barking and Dagenham MP Jon Cruddas. We hope this will stimulate similar discussions in the north and elsewhere, in the run up to the parliamentary debate in the New Year.

Matt Cavanagh is an Associate Director at IPPR. Follow him on twitter @matt_cav_