COUPLES who have more than two children are being “irresponsible” by creating an unbearable burden on the environment, the government’s green adviser has warned.

Jonathon Porritt, who chairs the government’s Sustainable Development Commission, says curbing population growth through contraception and abortion must be at the heart of policies to fight global warming. He says political leaders and green campaigners should stop dodging the issue of environmental harm caused by an expanding population.

A report by the commission, to be published next month, will say that governments must reduce population growth through better family planning.

“I am unapologetic about asking people to connect up their own responsibility for their total environmental footprint and how they decide to procreate and how many children they think are appropriate,” Porritt said.

“I think we will work our way towards a position that says that having more than two children is irresponsible. It is the ghost at the table. We have all these big issues that everybody is looking at and then you don’t really hear anyone say the “p” word.”

The Optimum Population Trust, a campaign group of which Porritt is a patron, says each baby born in Britain will, during his or her lifetime, burn carbon roughly equivalent to 2½ acres of old-growth oak woodland - an area the size of Trafalgar Square.

The British population, now 61m, will pass 70m by 2028, the Office for National Statistics says. The fertility rate for women born outside Britain is estimated to be 2.5, compared with 1.7 for those born here. The global population of 6.7 billion is expected to rise to 9.2 billion by 2050.

Porritt, who has two children, intends to persuade environmental pressure groups to make population a focus of campaigning.

“Many organisations think it is not part of their business. My mission with the Friends of the Earth and the Greenpeaces of this world is to say: ‘You are betraying the interests of your members by refusing to address population issues and you are doing it for the wrong reasons because you think it is too controversial,” he said.

Porritt, a former chairman of the Green party, says the government must improve family planning, even if it means shifting money from curing illness to increasing contraception and abortion.

He said: “We still have one of the highest rates of teenage pregnancies in Europe and we still have relatively high levels of pregnancies going to birth, often among women who are not convinced they want to become mothers.