The UN organisation in charge of global climate change negotiations is backing the fast-growing campaign persuading investors to sell off their fossil fuel assets. It said it was lending its “moral authority” to the divestment campaign because it shared the ambition to get a strong deal to tackle global warming at a crunch UN summit in Paris in December.

“We support divestment as it sends a signal to companies, especially coal companies, that the age of ‘burn what you like, when you like’ cannot continue,” said Nick Nuttall, the spokesman for the UN framework convention on climate change (UNFCCC).

The move is likely to be controversial as the economies of many nations at the negotiating table heavily rely on coal, oil and gas. In 2013, coal-reliant Poland hosted the UNFCCC summit and was castigated for arranging a global coal industry summit alongside. Now, the World Coal Association has criticised the UNFCCC’s decision to back divestment, saying it threatened investment in cleaner coal technologies.

Several analyses have shown that there are more fossil fuels in proven reserves than can be burned if catastrophic global warming is to be avoided, as world leaders have pledged. Divestment campaigners argue that the trillions of dollars companies continue to spend on exploration for even more fossil fuels is a danger to both the climate and investors’ capital.

“Everything we do is based on science and the science is pretty clear that we need a world with a lot less fossil fuels,” Nuttall told the Guardian. “We have lent our own moral authority as the UN to those groups or organisations who are divesting. We are saying ‘we support your aims and ambitions because they are fairly and squarely our ambition’, which is to get a good deal in Paris.”

The UN secretary general, Ban Ki-moon, sent a related message to investors in November, saying: “Please reduce your investments in the coal- and fossil-fuel-based economy and [move] to renewable energy.” But he stopped short of backing the divestment campaign itself.