Every £4 spent on family planning over the next four decades would reduce global CO2 emissions by more than a ton, whereas a minimum of £19 would have to be spent on low-carbon technologies to achieve the same result, the research says.

The report, Fewer Emitter, Lower Emissions, Less Cost, concludes that family planning should be seen as one of the primary methods of emissions reduction. The UN estimates that 40 per cent of all pregnancies worldwide are unintended.

If these basic family planning needs were met, 34 gigatons (billion tonnes) of CO2 would be saved – equivalent to nearly 6 times the annual emissions of the US and almost 60 times the UK’s annual total.

Roger Martin, chairman of the Optimum Population Trust at the LSE, said: “It’s always been obviously that total emissions depend on the number of emitters as well as their individual emissions – the carbon tonnage can’t shoot down as we want, while the population keeps shooting up.”

UN data suggests that meeting unmet need for family planning would reduce unintended births by 72 per cent, reducing projected world population in 2050 by half a billion to 8.64 million.

The research is published on the day that the Government’s climate change advisers, the Climate Change Committee, warned households and industry that a planned 80 per cent reduction in emissions are likely to prove insufficient.