In a speech today to environmental leaders, David Cameron said: "Today, I want to tackle an argument that seems to be as cyclical as the economy. The argument that when times are good, we can indulge ourselves with a bit of environmentalism - but when the economic going gets tough, the green agenda has to be dropped.

"According to this argument, protecting the environment is a luxury rather than a necessity - and it's a luxury we just can't afford in an economic downturn. I want this generation to be the one that bucks that trend: to be the generation that finds a way to combine economic, social and environmental progress.

"We will only do that if we develop a strategy for realistic environmentalism - not ignoring economic realities and just pressing on regardless but understanding economic realities and using them as a spur to innovation and imagination. "I understand that right now the cost of living is the number one concern for Britain's families. And I understand, with that backdrop, why people might think fighting climate change seems a costly diversion. But those who say we've got to choose either the environment or the economy, who say, "look, we can't tackle climate change. Going green will cost too much when the cost of living is already too high. Tough emissions targets will damage our industry and business…" they've got it exactly wrong.

"The truth is: it's not that we can't afford to go green - it's that we can't afford not to go green. When oil is moving towards $140 a barrel, when families are being hit hard every time they pay their gas bill, fill up their cars or do the weekly shop, are you telling me we shouldn't - we can't - go green? We've got to.