Ask an environmentalist in private what the world's biggest problems are and they will probably talk of climate change and human population growth, before moving on to disappearing species and habitats, deforestation, chemical pollution, resource wars, energy shortages and globalisation, to name just a few. Ask that same question in public and the odds are you will get one omission from that list - population growth.

Ask an environmentalist in private what the world's biggest problems are and they will probably talk of climate change and human population growth, before moving on to disappearing species and habitats, deforestation, chemical pollution, resource wars, energy shortages and globalisation, to name just a few. Ask that same question in public and the odds are you will get one omission from that list - population growth.

The UK population is now rising faster than at any time since the baby-boom years of the mid-20th century. Figures published last month suggest that some 300,000 new Britons appeared in our midst last year, equivalent to a city the size of Cardiff - equivalent also, at present rates, to some 125,000 new houses, over 150,000 vehicles, a lake nearly a third the size of Windermere in annual water abstractions, and roughly 1.6 million hectares (over four Hampshires) in terms of global ecological footprint.

By 2031, the Government projects, our population will have risen by 10 per cent to almost 66 million - nearly six million more - while calculations by the Optimum Population Trust suggest that at current rates there will be 10 million more Britons, equivalent to nearly one and a half Londons, by 2050. The UN, meanwhile, says global population will increase from 6.5 billion to 9.1 billion by mid-century.

The UK's landmass, of course, is getting no bigger, nor are its lakes and rivers becoming more numerous or productive. As it is, three quarters of us think Britain is overcrowded, record numbers are emigrating - driven clearly, as survey evidence shows, by a perceived decline in quality of life.

Not surprisingly, many of us are in for a summer of water shortages, almost certainly a foretaste of the future, and we're also involved in bitter arguments about land for housing. And this, of course, is without mentioning energy gaps, the peaking of oil, greenhouse gas emissions, or the unappetising choice between wind turbines and nuclear power.

If the environment was the Great Unmentionable of the election campaign, population is its younger, and even more forgotten, sibling - the environmental issue that dare not speak its name. Pressure groups steer well clear of it, politicians think it's a vote loser, and the Government hasn't been interested since the 1970s.

Perhaps the clearest example of this came recently when the charity Population Concern, which began life in the 1970s, changed its name to Interact Worldwide - an innocuous designation aimed at countering the chilling effect the words "Population" and "Concern" had on potential donors. Under the old name, researchers concluded, the group "did not have a long-term survival rating".

Why aren't we talking about population? One reason is its almost unique capacity to offend just about every shade of opinion, from the neo-cons and evangelicals, who see it as an attack on liberty or a promotion of contraception/abortion, to the left, for whom multiculturalism has achieved iconic status - and who therefore view any questioning of immigration, currently responsible for some 84 per cent of projected UK population growth, as tantamount to racism.

The perception has meanwhile arisen that the issue somehow conflicts with women's rights - "concern", the Interact researchers found, was treated as synonymous with "control". And when an air of controversy attaches to an issue, the first to take flight are the big corporate and charitable donors, without whom much civil society campaigning in the UK would cease to exist.

It may also, of course, be too "obvious" - and, as such, unpromising terrain for lobbyists seeking new angles or scientists in pursuit of research funding. Another way of saying this is that although population growth is implicated in every environmental problem we face, so is affluence and lifestyle and so too are our technologies. In the formulation by the population ecologist Paul Ehrlich, you multiply all three factors (population, affluence, technology) to arrive at an overall measure of human impact on the planet. Unfortunately, people invariably use this formula as an excuse for ignoring population - far easier to concentrate on lifestyles and technologies.

This has always been a cop-out and today, as affluence grows and technologies proliferate, it's beginning to look criminally irresponsible. Human numbers may turn out to be the part of the Ehrlich equation most amenable to reduction, and the reduction of which will be most compatible with individual freedom. Instead, when confronted with declining birth rates in parts of the developed world, governments have begun to argue that we need more people, not least immigrants, for economic reasons - to maintain growth and pay for our pensions.

A moment's thought will tell you that economic growth, if built on declining quality of life and the exhaustion of natural resources, is a social and environmental mirage. It's also a demographic nonsense - since all new additions to the population, whether from natural increase or immigration, eventually grow old and become part of the dependency "problem" they were meant so solve.

What's even more disturbing, however, is the pattern of denial. By accident or design, we seem to have expressly evolved structures that prevent us confronting the fundamentals of planetary survival. We could deal with the population issue if we began talking about it. At the moment, we're looking the other way.

The writer is an environmental writer and research associate with the Optimum Population Trust