Is Britain full? In the 1930s the Daily Mail campaigned against letting in German Jews on the basis that it was; since then the UK has accommodated more than 10 million extra people. But what was once a far-right trope is rapidly becoming conventional wisdom on both left and right. Even the supposedly impartial BBC now takes it as fact, if Monday's shockingly one-sided Panorama programme  was anything to go by.
The programme's starting point was projections by the Office of National Statistics  that, on current trends, Britain's population will reach 70 million by 2029, with two-thirds of the increase coming from new immigrants and their children. Such projections, the programme claimed, are accurate to within plus or minus 2.5 per cent.
Not so. The projection misleadingly extrapolates to the distant future the exceptional rise in immigration in the years before 2008, when the economy was booming unsustainably and the government has just let in workers from Poland and other east European countries. But now boom has turned to bust and the plummeting pound has devalued British wages, new arrivals from eastern Europe halved between 2007 and 2009, and many are going home. In the 12 months to mid-2009, immigration was already well below the ONS projection.
Previous projections have proved wildly wrong. In 1965, official statisticians reckoned Britain's population would reach 75 million by 2000. It turned out 16 million lower. And while short-term projections based on births and deaths may be reasonably accurate, longer-term estimates of future migration should be taken with a fistful of salt.
The programme's other premise was that a rising population -- due to immigration -- was overwhelmingly a bad thing. Except for a 10-second clip of me putting a positive case for immigration, the message was unrelentingly negative. Immigrants were said to contribute little to the economy and strain scarce resources -- seats on trains, hospital beds, social housing, living space, you name it.
This is misanthropic nonsense. Each of us contributes to society in all sorts of ways -- and immigrants are no exception. On the contrary, newcomers tend to chip in more than most. Research by Christian Dustmann  and his team at University College London shows that newcomers from eastern Europe paid 37 per cent more in taxes than they received in benefits and from public services in 2008-09, while people born in Britain paid in 20 per cent less than they received. In other words, recent migrants are not a drain on the welfare state, they are helping to pay for it -- while many more migrants help to provide public services, as doctors, nurses or cleaners in the NHS, for instance. So if public services are failing to respond fast enough to a changing population's needs, blame the government, not foreigners. After all, if a Brit moved from Carlisle to Cardiff to take up a new job and there wasn't a place for their child at the local school, who would you blame?
Yes, a House of Lords select committee report chaired by the Conservative politician John Wakeham concluded that the economic benefits of immigration to the existing UK population were probably small -- and how could the great and the good ever be wrong? But it ignored the extra dynamism that a diverse mix of people sparking off each other brings, and all the new ideas, technologies, businesses and economic growth that this generates. Twenty-two of Britain's 114 Nobel laureates were born abroad; Tesco, Marks & Spencer, easyJet and many other successful companies were founded by immigrants and their children; and new arrivals of all cultural backgrounds are twice as likely to start a business as people born in Britain, as I explain at greater length in my new book, Aftershock: Reshaping the World Economy After the Crisis , which is out on 6 May.
Just look at Silicon Valley: Google, Yahoo!, eBay, YouTube, and many others, were all co-founded by immigrants who were not selected by some clever points system but arrived in the US as children. Anyone who doubts the contribution that newcomers make should ask themselves whether a global city such as London would be half as successful without a constant influx of new people from around the country and the world.
Immigrants' contribution to this country is positive and large. But surely they also strain scarce resources? Not necessarily. The main problem is not too many people, it's a lack of investment. Paris is more densely populated than London yet its excellent Metro is less crowded than our Tube. The Netherlands has more people per square mile than Britain, yet there is plenty of space on Dutch trains. Roads are clogged mostly because car ownership has risen as people have got richer; central London's congestion charge has unblocked them.
Social housing is scarce not because newcomers are taking all of it -- in most cases they are not entitled to it -- but because lots of homes have been sold off and very few new ones built. House prices are sky-high because demand has risen as people have got richer and more single people want to buy their own place, while planning restrictions limit the supply of new developments, and Britons are so addicted to property speculation that they are willing to borrow ever larger sums to get on the "property ladder".
Britain is not full up. Three-quarters of it is agricultural land. Contrary to the belief that it is about to be concreted over, building 3 million new homes at the government's target density would take up only 0.3 per cent of the UK's land area -- and much less if houses were built on brownfield sites. Nor is rising population density necessarily a bad thing. Most people in Britain choose to live in cities and suburbs rather than rural areas. The most densely populated place in the country is Kensington and Chelsea, which is hardly a hellhole. After all, that's where David Cameron chooses to live.
Philippe Legrain is a visiting fellow at the London School of Economics' European Institute and the author of Immigrants: Your Country Needs Them  (Little, Brown).