It's the one issue no environmentalist organisation wants to talk about. Population. Thirty years ago, when international concern first began to mobilise about the planet's future, it was the pre-eminent question, but now you're hard put to get a straight answer. Does the UK need population management? Does the world need it?

This is one of those issues that is regarded by many privately as common sense but rarely gets a public airing. Of the environmental organisations I managed to contact, all acknowledged that it was frequently brought up by the public in meetings and letters. Yet all said they did not campaign on the subject and had no position on it. It seems that there is a worrying disconnect between a generally accepted consensus among those who shape the national conversation about the environment and their audiences, who either are much less certain or believe that, if the planet's resources are being grossly depleted, there are just too many of us about.

Too many people. That is certainly the impression from studying the maps published today by the Campaign to Protect Rural England, which chart how fast the areas of the country undisturbed by urban development, roads or other noise are disappearing. Since the 60s, whole chunks of England have been broken up into small fragments, absorbed into a dense network of towns, cities and major roads.

The maps reinforce what people experience. You try getting away from it all in England, and you are tangled in traffic jams, shoe-horned into campsites, followed by the whine of motor-bikes and the roar of traffic even up on the hills. We live in a crowded island - a truth that it has become unacceptable to acknowledge because of the unpleasant associations it brings with it.

But England is now the second most densely populated country in Europe, after Belgium, and at current rates of increase it could be second only to Bangladesh in the world by 2074. There are those who argue that there's no need for alarm, and that we can concentrate development in brownfield sites to accommodate all the millions of extra homes needed. But how many more people can you squeeze into cities that already seem to be choking under the weight of their population density - the buses and trains packed, the streets clogged and the parks on a Sunday afternoon teeming with people.

It's not surprising that environmental organisations fight shy of getting into this subject. It embroils them in a host of deeply emotive and difficult debates. Immigration for one. Most of the UK population growth in the next few decades will be attributable to immigration. Should we have a balanced migration policy with a net zero increase? Given how many British-born are emigrating to Australia, the US, Spain and France, it would still allow us to maintain our international responsibilities to provide asylum. But it wouldn't allow us to absorb the same quantities of cheap east European labour that have subsidised our economic growth.

Population management is just as emotive. People quickly bristle at the idea of any government telling them how many children they can have. The whole policy area of population was given a bad name by India's enthusiasm in the 70s and 80s when government programmes ensnared uncomprehending young men into having vasectomies. But should the UK government pursue a policy of persuasion, a Stop at Two campaign, to bring people's attention to the carbon footprint of having lots of children? If it did, would it work? Internationally, population policy has been crippled by US and Vatican opposition on abortion and contraception. Have they managed to bully environmental organisations into this awkward silence?

When challenged, environmentalists have coherent arguments to defend their retreat from the population debate. They insist that the pressure on the earth's resources - its water, forests, soil fertility - and carbon emissions are all about consumption and lifestyle, not about sheer numbers of human beings. They rightly point out that the average American produces some 20 tonnes of carbon a year while some of those living in areas of the world with the fastest growing populations, such as Africa, produce a tiny fraction of that kind of carbon footprint. They insist that the earth can support the 9 billion now predicted by 2050 (the increase in the next 40 years will equate to roughly what the entire global population was in 1950) if everyone is living sustainable lifestyles. The focus of campaigning must stay on the consumption patterns of the developed world, rather than on numbers of people.

But there is growing disquiet that it's not an either/or. As the environment finally gets the prominence it deserves, some environmentalists are prepared to assert that population management has to be on the agenda. Christopher Rapley, the director of the Science Museum, has spoken out on the subject; Jonathon Porritt, chair of the government's Sustainability Development Commission, admits it is "tough territory" but argues that "it is intellectually unjustifiable" for the environmental movement not to address it. He wants to see a UK population policy that covers both family planning and immigration, aimed at long-term population decline. That would mark a dramatic shift in policy. In particular, he rejects the oft-cited need to keep up the birth rate to pay for pensions. But his attempts to get the government to engage have got nowhere.

As Porritt ruefully admits, his position lands him in some unsavoury company. The Optimum Population Trust proposes some batty ideas such as government campaigns on the unattractiveness of parenthood. And it gets much worse. As is often the case where there is a disconnect between public debate and popular sentiment, the British National party (BNP) is stepping in to grab the territory. It argues that "our countryside is vanishing beneath a tidal wave of concrete", "immigration is creating an environmental disaster" and Britain could become "a tarmac desert".

The BNP is peddling alarmist nonsense. Only 8% of the land of this country is built on, but, as a Mori poll commissioned for Kate Barker's review of land use for the Treasury showed, it doesn't feel like that: those polled put the figure at 50%. This sense of crowdedness and the resentment it can generate needs a grown-up debate. That means talking about consumption patterns and population numbers. It includes pointing out that consumer trends - such as our taste for mobility, the move to smaller households and multiple bathrooms - squeeze the space and resources we share on a small island. But it would also include discussion of the environmental impacts of migration and family size. There's no point giving up your meat and your car, recycling your rubbish and producing lots of children. The challenge is to have that debate while steering well clear of racism - or of the authoritarianism that lurks in the background of environmentalism.

m.bunting@theguardian.com