A top scientific academy on Thursday called on June's Rio Summit to tackle population growth and voracious consumption that are placing Earth's resources under intolerable strain.

"The 21st century is a critical period for people and the planet," the Royal Society, the world's oldest science academy, said in a report ahead of the June 20-22 UN gathering.

Demography, it said, can no longer be sidelined or treated as separate from the environment or the economy.

"The world now has a very clear choice," said leading British scientist Sir John Sulston, who led the report.

"We can choose to rebalance the use of resources to a more egalitarian pattern of consumption, to reframe our economic values to truly reflect what our consumption means for our planet and to help individuals around the world to make informed and free reproductive choices."

"Or," he said, "we can choose to do nothing -- and to drift into a downward vortex of economic, socio-political and environmental ills, leading to a more unequal and inhospitable future."

Earth's population is expected to roughly triple by 2050 compared to a century earlier.

It stood at three billion in 1950, reached seven billion in 2007 and is likely to reach around 9.5 billion by 2050, according to UN estimates.

Despite this surge, the issue of population growth is dormant in international politics. In the "zero draft" communique being prepared for the Rio summit, there are few references to demography.

The Royal Society report, People and the Planet, said population growth rates were slowing or going into reverse in many countries but were predicted to remain high in least-developed economies.

Rich nations are by far the biggest users of resources per capita, and their wasteful practices are spreading to emerging economies with giant populations, it said.

This combination of a growing global population and an accelerating pattern of consumption has alarming implications, the report warned.

"The Earth's capacity to meet human needs is finite," it said.

For example, a child in the developed world uses between 30 and 50 times as much water as his or her counterpart in a developing country.

Per capita emissions of carbon dioxide in rich countries are up to 50 times higher than in poor ones.

Average consumption of food has risen on average by 15 percent in calorie terms over the past four decades, yet nearly a billion people remain poorly nourished.

The report put forward nine recommendations for tackling the intertwined problems.

It demanded that 1.3 billion people living on less than $1.25 (about one euro) a day -- be brought out of absolute poverty.

"The most developed and the emerging economies must stabilise and then reduce material consumption levels," through investment in energy efficiency and clean technologies, steering them onto a path of sustainable development, it said.

Leaders should urgently commit to programmes of voluntary contraception and education, which are big factors in bringing down fertility rates.

And they should consider the impact of demography when they come to making economic or environmental decisions.

"Demographic changes, and the influences on them, should be factored into economic and environmental debate and planning at international meetings, such as the Rio+20 conference," it said.

Around 100 heads of state or government are expected for the conference, which marks the 20th anniversary of the Earth Summit that declared the environment a priority, according to Brazilian sources.