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Alan Johnson, the Home Secretary, said recently that he did not lie awake at night worrying that the population of the United Kingdom might rise to around 70 million, probably in his own lifetime. When Mr Johnson was born, in 1950, there were 50 million people in this country. Today, there are 61 million. Taken alone, England is the fifth most densely populated large country in the world – behind Bangladesh and South Korea and well ahead of Japan or China.So, at what projected population would the Johnsonian slumbers be interrupted: 80 million, maybe? 90 million? Or does Mr Johnson calculate that by the time it reaches 100 million he will not be around to worry about the consequences, so why should he care?The latest population estimates from the Office for National Statistics confirm that we are seeing the fastest growth in population since the post-war baby boom. In the 1950s and 1960s, however, a high birth rate was considered something to be encouraged because a rising population meant more people would be available to work, and a bigger economy meant greater wealth. And so it has proved. Indeed, it has always been the case that in order to have economic growth it is necessary to have more people. Countries whose populations stagnate and decline are countries with no future.But this was not always considered a truism; and there must come a point where it is a falsehood. It is a question of knowing when the population becomes too high and what to do about it.In the 18th century, Thomas Malthus imagined that a population of about 10 million was unsustainable. His 1798 essay On the Principle of Population predicted war, pestilence and starvation as a result of overcrowding. Yet at the time, Britain was on the cusp of an agrarian and industrial revolution that would provide both a motor for rapid population growth and the means to feed the extra people. Between 1800 and 1850, the population of Britain doubled.In the 1960s, the population doomsayers were at it again. Paul Ehrlich's book The Population Bomb predicted that by the following decade, millions would have starved to death, as our natural resources and food ran out. Ehrlich was right: the world's population has indeed exploded, up by more than 60 per cent over the past three decades. But he grossly underestimated the capacity of the world to cope, thanks to the discovery of new energy reserves and spectacular improvements in agricultural productivity – the so-called Green Revolution.Today, the world's population is 7 billion and is growing by 78 million a year. Yet the world's resources have still not run out: recent famines have been caused largely by war; malnutrition has declined; and fewer people starved to death in the final quarter of the 20th century than in the last 20 years of the 19th.A bigger population is an engine of economic growth; yet greater prosperity tends to lead to a fall in the birth rate, as is the case in Europe, where it is below replacement levels in most countries and falling. This is a big worry for countries such as Italy and France – and for Germany, now Europe's most populous nation but, according to UN estimates, on course to be overtaken by the UKby 2050.This is the most startling aspect of the population projections: not that the number of people might reach 69 million, or 72 million – but that it is going up at all. Until relatively recently, it was predicted that the UK population would stabilise at under 60 million; indeed, the planning of governments since the 1970s has all been predicated upon a static population.Infrastructure, the capacity of schools and hospitals – all the utilities needed to make a modern, prosperous country work have to be planned well in advance. That is one reason why we have statisticians and actuaries trying to predict the future. And that is why Alan Johnson should be lying awake at night – because none of this was anticipated.There are two reasons why the population is growing. One is that we are living longer, so there are more older people than ever before. Child mortality rates are historically low, which means parents have small families, which they did not when the chances of losing offspring to disease were high.But the principal driver of population growth is immigration, directly through the numbers arriving every year and indirectly through the higher birth-rate among first- and second generation foreign-born mothers.This is what makes population growth in Britain today so different. Once, it was fuelled by a high birth rate that saw a rapid rise in the indigenous population; now it is changing the nature of the nation itself, making it far more diverse ethnically and culturally. Some people are happy with that; others are not. But there is no use pretending it is not happening.The most likely forecast based on current trends is that the population will grow to 71 million in 2031 and to 85 million in 2081, but if birth rates rise more quickly than expected, immigration remains high and people live longer, this figure could be 108 million.But do we have any choice if we want to remain a viable and economically strong nation? Big economies have big populations – look at America, a nation founded entirely on immigration, where there are now more than 300 million people.Over the next 25 years, the population of the UK aged 65 and over will rise by 63 per cent to 15.8 million. We will need more young people in work to sustain an increasingly elderly nation. Is immigration the answer? After all, immigrants themselves grow old and become pensioners. Also, their own fertility patterns start to match those of the country they settle in. And while their children are growing up they, too, are a burden on schools and hospitals.There are now 61 dependants for every 100 people of working age. By 2031, taking into account the impact of immigration, there will be 64. So the dependency ratio, as it is called, is worsening, even with record immigration.The impact of population growth is felt especially severely in London and the South East. We are such a top-heavy nation that migration, both internally and from overseas, tends towards the capital and its environs. But it has always been thus. People battling to work on crowded buses and trains every morning may find it hard to believe, but London's population is lower than it once was. It peaked at around 8.6 million in 1939 before beginning a steady decline as people moved into the suburbs and beyond. By 1990, it was down to around 6.5 million. Immigration, though, is pushing it back up.We were clearly not prepared for this, either logistically nor culturally. If the Labour Party was intent upon an immigration-fuelled population boom, it was not something it put to the nation in 1997. Ministers now seek to make a virtue out of something that was simply never planned for and that lax immigration policy allowed to happen, deliberately or not.What will the impact be on the quality of life in Britain of an additional 10 million people in just 20 or so years? In the past, such rapid growth has resulted in long-term prosperity, though it has often been accompanied by short-term privation as in the city slums of Victorian England.Roger Martin, chairman of the campaign group the Optimum Population Trust, does not think the country can sustain such growth any longer. "Britain's population increase is out of control and we are on course for a high-density, low-quality future where overcrowding and congestion are the norm and resource shortages, particularly of vital commodities like water and energy, are ever more pressing."Every addition to the population pushes this country further from sustainability and nearer to a position of extreme environmental precariousness. This is a future nobody wants."But it is a future that we are going to get without a fundamental rethink in government policy. Far from enjoying a good night's sleep, Mr Johnson should be having nightmares.