Number 18 of the top 50 things to do to save the planet is to have fewer people on it, according to a recent Environment Agency report.

Sarah Lewis asks, can we really fit all these people on our planet?

The current global population of 6.6 billion people is predicted to rocket to a staggering 9.7 billion in the next 40 years, putting an unprecedented stress on our natural resources.

Yet while we are all busy changing our light bulbs and campaigning to ban plastic bags, there is a conspicuous silence hanging around the topic of sustainable family planning.

Chris Todd is an environmental campaigner and spokesperson for the South Downs Campaign. He says: "I don't think we should hide from debating uncomfortable truths.

"Population growth clearly is one of the factors which determines our impact on the Earth's ecosystem and therefore we should talk frankly about it. People have become afraid of speaking their mind because of a fear of being labelled as intolerant, sexist, racist, ageist - etcetera, etcetera .

"Population growth could wipe out any gains we make reducing the amount we consume - or certainly make it very much harder to achieve.

Therefore it has to be a part of the discussion and not ignored as some form of sacred taboo."

Friends of the Earth say they do not campaign on the matter of population, claiming the big issue is resource use: "The richest 20 per cent of people consume 86 per cent of the Earth's resources and have the lowest birth rates. G8 countries make up just 13 per cent of the world's population, yet emit nearly half of all greenhouse gases."

But Green Party parliamentary candidate for Brighton Pavilion, Caroline Lucas MEP disagrees. "The stark fact is there's a direct relation between the total emissions we produce and how many of us there are," she says. "By simple arithmetic, if a population halves its emissions but doubles in size, it won't make any difference at all."

The idea of reducing or controlling the population may be distasteful but on a planet with finite resources and an exponentially growing number of people something, at some point, has to give. Yet, even in the face of this, our rampant and inequitable consuming shows little sign of abating.

Chris says: "At present we are not able to feed the world's population adequately, yet we produce enough food to do so. That is a clear failure of our current political, economic and moral structures. With the world's population set to rise significantly over the next century, if we can't cope now, how are we going to cope then?"

In East Sussex, the current population of 509,000 is expected to increase to 519,000 by 2026, despite a death rate which outstrips the birth rate 12.9 to 9.2 per thousand people, due to population movement.

"Immigration is very much linked with racism, which has held the debate back," says Chris. "If everyone lived like we do in the South East, we would need three to four planets to support us, which, of course, is impossible.

"By allowing or encouraging high levels of immigration we are fuelling the problem not improving it because when people come here to work they are, naturally, going to start living our unsustainable lifestyle, too."

There are other issues which come with this expected growth. The South East Plan proposes a further 11,000 homes should be built in Brighton and Hove by 2026 and, while sustainability is a feature in the plan, the result is likely to be severe pressure on our natural resources, such as water, of which we already have one of the highest per capita consumption rates in the country. There is also the problem of space - can a city hemmed in by the sea and South Downs accommodate any more without compromising quality of life and the future of the South Downs National Park?

Chris says: "Without increasing our physical footprint and spreading out onto the Downs, which could cause huge damage in itself, we risk undermining the quality of the city and other settlements in Sussex if we allow too much development and in the wrong places."

If in Sussex the problem is the movement of people, elsewhere birth rates are skyrocketing.

According to the UN, there are 78 million people added to the world every year, yet there are 200 million women who want to control their fertility but have no safe and effective access to contraceptive services.

Caroline says: "The fastest way to get a stable population is to have a fair, secure and educated one - and we must be striving for that.

"We need a major investment in family planning so women can choose their family size. There is impressive evidence of the results carefully designed, culturally sensitive programmes can achieve - and of the benefits it brings to women and their communities, even before thinking about global population impact."

In the Sixties and Seventies, when environmentalism was in its infancy, population was a key issue for all the major campaign groups. Even as recently as 1994, Oxfam published a paper entitled World Population: The Biggest Problem Of All.

But in 2007, to call for such temperance or even frank discussion runs too great a risk of upsetting the other values environmentalists so typically identify with: human rights, gender equality, race, immigration and, above all, individual choice.

Caroline says: "We've got to stop being paralysed by the sensitivities the population question naturally taps into and recognise there are actually valid ways to address it which could bring great benefits.

"I believe the decisions we make relating to family issues, including how many children we have, must be left up to individuals without any interference whatsoever by the state.

"But devoting sufficient resources and efforts to reproductive health and family planning services brings genuine win-wins in terms of community development and women's rights, as well as smaller populations."

What the experts are saying:

NICK REEVES, executive director Chartered Institution of Water and Environmental Management "Scratch the surface of any environmental problem and it reveals population growth, and the way we live our lives, as the root cause. The need for a population policy has never been more urgent. While governments continue to see big populations as an indicator of economic strength, with a place at the top table of the UN guaranteed, the population problem will escalate and lead to environmental catastrophe."

AISHA HANNIBAL, coordinator, Brighton Peace and Environment Centre
"We live in a world of abundance with more than enough resources to feed, house, and clothe all the world's people. The issue is not overpopulation but poverty, war, lack of health provision and education. Rather than globalising unfair trade regulations we should find methods to globalise economic justice and live within our fair share rather than our current demand in the West."

PAUL HUTCHINGS, Brighton World Development Movement
"Rather than focusing on population growth, our focus should be on use and distribution of resources. Yes, environmental issues would influence my choices with children but I don't think we should be telling people how many children they should or shouldn't have. We should have policies which are about the sensible use of resources and the use of green technology.

More of an issue than population growth is population movement.

That is really something we will have to deal with in the future."