JONATHON PORRITT, one of Gordon Brown’s leading green advisers, is to warn that Britain must drastically reduce its population if it is to build a sustainable society.

Porritt’s call will come at this week’s annual conference of the Optimum Population Trust (OPT), of which he is patron.

The trust will release research suggesting UK population must be cut to 30m if the country wants to feed itself sustainably.

Porritt said: “Population growth, plus economic growth, is putting the world under terrible pressure.

“Each person in Britain has far more impact on the environment than those in developing countries so cutting our population is one way to reduce that impact.”

Population growth is one of the most politically sensitive environmental problems. The issues it raises, including religion, culture and immigration policy, have proved too toxic for most green groups.

However, Porritt is winning scientific backing. Professor Chris Rapley, director of the Science Museum, will use the OPT conference, to be held at the Royal Statistical Society, to warn that population growth could help derail attempts to cut greenhouse gas emissions.

Rapley, who formerly ran the British Antarctic Survey, said humanity was emitting the equivalent of 50 billion tons of CO2 into the atmosphere each year.

“We have to cut this by 80%, and population growth is going to make that much harder,” he said.

Such views on population have split the green movement. George Monbiot, a prominent writer on green issues, has criticised population campaigners, arguing that “relentless” economic growth is a greater threat.

Many experts believe that, since Europeans and Americans have such a lopsided impact on the environment, the world would benefit more from reducing their populations than by making cuts in developing countries.

This is part of the thinking behind the OPT’s call for Britain to cut population to 30m — roughly what it was in late Victorian times.

Britain’s population is expected to grow from 61m now to 71m by 2031. Some politicians support a reduction.

Phil Woolas, the immigration minister, said: “You can’t have sustainability with an increase in population.”

The Tory leader, David Cameron, has also suggested Britain needs a “coherent strategy” on population growth.

Despite these comments, however, government and Conservative spokesmen this weekend both distanced themselves from any population policy. ”