The northern ticket hall of King’s Cross St Pancras tube station is supposed to be a wonder of the London underground, with its expanses of gleaming floor and high-ceilinged walkways that would be wide enough for cars. In 2008, the tube’s then managing director, Tim O’Toole, assured the London Evening Standard that, with the new hall, which cost £395m, “the underground station complex will ... be capable of handling all the extra demand predicted for years ahead”.

It has not worked out like that. In the morning rush hour, the pedestrian tunnels are packed. Every few months, the whole complex becomes so congested that it has to be evacuated.

In big and small ways, London feels more crowded these days: parks fill to pop-festival density on every sunny weekend; West End shoppers walk in the road because there is no space left on the pavement; mobs of kerbside drinkers dwarf poky pubs in Soho; East End evenings of the most esoteric music sell out; rush hours last all day. People have complained about London overcrowding for centuries, but the current situation is new. Last year, the capital’s population reached an all-time peak of more than 8.6 million. By 2050, it is forecast to be 11 million, and possibly as high as 13 million.

The UK population is growing unusually fast, too. At the present rate of progress, the Office for National Statistics expects it to swell by 4.6 million during the 2010s – “the biggest growth in the last 50 years”. In 2014, the latest year for which figures are available, the UK had almost 65 million inhabitants, its greatest ever total. It is predicted to be home to more people than France by 2030 and more people than Germany by 2047, which would make this much smaller land mass the most populous country in Europe.

The population of Ashford in Kent grew by nearly 30% between 1984 and 2009. Photograph: Alamy

This accelerating change seems all the larger for being unexpected. “I don’t think anybody saw the population turn coming when it started in the 90s,” says Danny Dorling, a demographer and professor of geography at the University of Oxford. “In the UK, we don’t tend to notice these kind of social transformations that fast.”

But now we have noticed this one. Population growth has become a background anxiety, or a source of deafening acrimony, in a vast range of political and social controversies: from shortages of school places to the pressure on the NHS; from airport expansion to the housing crisis; from road congestion to the north-south divide (London and the south-east are growing faster than most other regions); from our ability to accept refugees to our approach to multiculturalism; from our low-wage economy to the state of the environment; and the most fraught domestic issue of all, immigration.

Our expanding population is almost always talked about in negative terms. “People haven’t yet got to the stage of thinking about anything but the problems,” says Conservative MP Damian Green, who served as minister for immigration from 2010 to 2012. “The public do sense the crowdedness,” says Sir Andrew Green (no relation), the founder of MigrationWatch UK, an influential pressure group that supports the curbing of immigration. (Green was made a peer by David Cameron in 2014.) “Most people don’t want a more populous country. They don’t want the disturbance of large numbers of people coming. I don’t think that either recent Conservative or Labour governments have focused on the scale of the increase that is coming down the track.”

In 2010, a cross-party group – including the Labour MP and poverty expert Frank Field, the economic historian Lord Skidelsky and the former Archbishop of Canterbury Lord Carey – issued a “Declaration on Population”. It warned that a UK population of 70 million – which will arrive about 2030, based on current trends – “would be seriously damaging to the future harmony of our society.” Days later, David Cameron announced his ill-fated plan to reduce net immigration.

Ashford’s residents are divided over the town’s fast growth, but planner Richard Alderton says ‘there is still lots of lightly used land’. Photograph: Scott Barbour/Getty Images

But is alarm the right response to the population boom? Jonathan Portes of the National Institute of Economic and Social Research thinks not. “Population is not well discussed in Britain,” he says. “Our self-image is an old and constrained country. We find it hard to be positive about population growth. But it has boosted economic growth. It has made austerity less painful, by increasing total employment and tax revenues. And congestion, pressure on services – they’re considerably easier to cope with, from a collective point of view, than the opposite problems. We’ve forgotten what depopulation feels like.”

Portes grew up in London in the 70s. The postwar baby boom was ending. The arrivals of modern contraception and feminism were making families smaller. The British economy was entering a troubled phase. Millions of manufacturing jobs were disappearing from cities. For many Britons, expectations about the future were diminishing.

Between 1975 and 1978, the UK population fell. In 1982, it dropped again. “The population of inner London fell by 20% in the 70s,” says Portes. “Many people said London was basically doomed. It was going to go the way of Detroit. Inner London would become wasteland.”

The idea of leaving the country became a common part of British conversations and life decisions. In the mid-70s, the number of people emigrating rose sharply. In 1974, the Labour foreign secretary, Jim Callaghan, told the cabinet, “If I were a young man, I would emigrate.” Soon afterwards, Margaret Thatcher, then struggling as Tory leader, told the novelist Kingsley Amis that she was considering sending away her children to start “careers in Canada”.

The consequences of depopulation could be bleak: boarded-up houses; miles of urban dereliction; dwindling investment and passenger numbers in and on public transport. Indeed, in the 70s and 80s, Londoners feared empty, potentially threatening tube carriages as much as they fear overcrowded ones today.

London’s station have not always been packed with commuters, as this picture of Liverpool Street station from 1975 shows. Photograph: Evening Standard/Getty Images

In some places, despite the recovery of the population since, this emptied Britain still exists. Liverpool and Glasgow have barely half as many inhabitants now as they had at their peaks in the middle of the 20th century. This can give them an ease, freedom and affordability that southern cities lack, but there can also be something unsettling about places where so many former residents voted with their feet.

The population boom elsewhere in Britain started in the mid-90s. There were international factors: the end of the cold war, a more interwoven global economy and the expansion of the EU. British universities sought more foreign students, and Mediterranean countries began to struggle economically. Social trends contributed, too, such as the tendency for immigrants to have more children than other Britons, the introduction of IVF treatment for older women, an overall improvement in the national fertility rate, a decline in the death rate and an improvement in life expectancy. The Blair and Brown governments also provided more support for families through tax credits and better childcare facilities. The rate of population growth doubled in the 90s and doubled again in the 00s.

One of the fastest-growing communities in England in this period was Ashford in Kent. Between 1984 and 2009, this unremarkable but well-connected town – 60 miles from London, 50 miles from France – grew by 29%. Currently, Ashford has 78,000 inhabitants. “By 2030,” says the council’s head of planning, Richard Alderton, “we’ll probably have 105,000.”

It’s not hard to find people in the town who think this expansion is a mistake. “This used to be marshland,” says Colin Wingrave, a long-term resident, gesturing at the endless cul-de-sacs of Park Farm South, one of several suburbs of narrow, detached houses with few amenities that have been built in a confusing patchwork around the centre of Ashford. Wingrave worked in a local electronics factory until it outsourced production to Hungary; he was forced to retire two years early. He says Ashford has “more and more houses and fewer good jobs”. But his objections to its growth go beyond the economic and the environmental. “It’s just not right,” he says. “You can’t put your finger on it.”

Blackhorse Road tube station in 1974. ‘In the 70s and 80s, Londoners feared empty, potentially threatening tube carriages as much as they fear overcrowded ones today.’ Photograph: Heritage Images/Getty Images

Alderton, who has been Ashford’s planner for 27 years, says the town divides into two broad camps. “Anecdotally, people who’ve lived here longer look a bit askance at the town growing quite so fast,” he says. “They don’t like the traffic congestion and they yearn for when we used to have lots of independent shops and a cattle market. That contrasts sharply with the incoming population: young families. They realise that more people means more spending power, more leisure facilities.”

The owner of the convenience store at Park Farm South is one of the latter group. He emigrated from Sri Lanka to London in the 90s and moved to Ashford six years ago. “I want Ashford to be big!” he says. “It was too quiet when I came. And I want to feel good about my town.” But what about all the traffic? “It’s not too bad. The schools are still OK. There are still green fields all around.”

For those who argue that the UK, and the south-east of England in particular, is “full up”, Ashford is an awkward case. The town centre is busy on a weekday lunchtime, but hardly overwhelmed. The foot traffic is younger and more multiracial than in less expansive Kent towns, with the occasional French accent. There is still lots of lightly used land – probably too much – occupied by former market sheds and temporary car parks. “We’ve succeeded in terms of population growth so far,” says Alderton, summing up the council’s approach. “But the town centre could do with more residents, with a bit more buzz.”

There are intermittent stories in the local press about migrant stowaways arriving in Ashford from the nearby Channel tunnel, but at last year’s borough elections Ukip earned an unspectacular 10% of the vote. “People complain constantly about the council’s growth plans,” says Damian Green, who for the last 19 years has been Ashford’s MP. “But we rub along.”

Beyond Ashford, too, the population boom has been going on for so long without a social meltdown that the pessimists have had to adjust their arguments. “There’s a certain amount of elasticity in society,” says Andrew Green. “Things don’t collapse. Instead, the balloon stretches thinner and thinner.” Even Green doesn’t think the UK’s population growth should be stopped: he favours “a steady but much lower rate of increase”.

He may be being realistic. In 2011, a Royal Commission on Demographic Change and the Environment concluded: “In practice, there is little government can do to have any real effect on the size of the population over the next 40 years.” The boom’s causes are too interconnected and powerful – and the British state insufficiently authoritarian – for our population trends to be set by Whitehall. The commission recommended instead that governments protect the UK by “improving resource use and influencing consumption patterns”.

Jonathan Portes points out that much of the UK is not crowded anyway. All population statistics are by definition slightly out of date and approximate, but while England has roughly 410 people a sq km – the second highest in the EU – Wales has only 150, Northern Ireland 135 and Scotland 70. Even heaving, stressful London is much less full of people than is widely supposed. “London is the lowest-density mega-city on the planet,” says Danny Dorling. “The densest part of London is four times less dense than Barcelona, a normal, well-planned European city that Britons all want to visit.”

Dorling argues that the UK’s “overpopulation problem” is really the product of poor land use and social division, of corporate wage squeezes and cuts in state provision. “We’ve managed to organise ourselves so that much of our daily lives is crowded. We have the smallest homes in Europe. Meanwhile, there’s lots of wasted space.” Inner London is increasingly taken up by the huge, little-occupied homes of the super-rich and empty investors’ properties – a less tatty, but in some ways more dysfunctional and depressing, form of urban emptiness than the rundown streets of the 70s and 80s. At least those had the potential to become spaces for community groups, poor immigrants or bohemians.

In the parts of London and other British cities that remain heavily populated, says Dorling, “our fear of each other makes it much harder to live together. Density is much harder when a society is very unequal. Look at Japan: it can be very dense because it’s very equal”. Yet, thanks to the media and popular culture, many Britons are more familiar with the space-hungry suburbs of the US and Australia than the reality of life in modest flats in Japan or Barcelona. Our aspirations often remain suburban. Meanwhile, inadequate infrastructure further amplifies our overpopulation anxieties. As Dorling says, “We have mainline railway stations with pavements outside them that are are only two metres wide.”

He thinks the population panic will pass. “I find it hard to believe that we’ll have this gloomy discourse on population in 20 years’ time.” Portes agrees: “You can build more schools and hospitals. Population redistribution is hard, but not impossible. You obviously can’t plonk people in the middle of nowhere, but we built new towns in the 50s. Why not build more within commuting distance of, say, Manchester?”

Damian Green, with his recent experience as an immigration minister, is less sanguine. In Whitehall, “most of the discussions were about bringing the [immigration] numbers down. But I’d rather be in a country people wanted to come to than leave. We are a more self-confident country now than in the 70s. If we get relatively bigger [than our rivals], it will increase that.” Does he think the UK has an optimum, or maximum, population? “No. If you try to guess a number, that becomes a target.”

Sooner or later, Dorling points out, the current rise will go into reverse. The British economy will enter a recession and cease to be so attractive to immigrants. The Mediterranean economies will recover. Even the civil wars in the Middle East and Africa, and the resulting refugee crisis, will end. At this point, the size of the British population will depend much more on our fertility rate, which is around 1.9 children a family – one of the highest in Europe, but lower than the 2.1 needed to keep a population stable.

In the end, perhaps only two things can be said for certain about population trends. Sooner or later, they make fools of those who offer dramatic forecasts. But people will keep making them. In 1960, in the US journal Science, a paper by the distinguished physicist and philosopher Heinz von Foerster and two colleagues declared, “Our great-great-grandchildren will not starve to death. They will be squeezed to death.” The paper was titled Doomsday: Friday, 13 November, AD 2026. See you in the northern ticket hall then?