US public relations giant Edelman said Tuesday it will no longer represent coal producers or take assignments that seek to deny climate change.

"We reserve the right to make a principles-based decision to not work on a particular category," an Edelman spokesman told AFP.

"Given that coal emits the most CO2 of any fossil fuel per unit of energy obtained, we made the category decision to not work for coal producers going forward. Furthermore, we do not accept client assignments that aim to deny climate change."

The firm's move was first reported by the British newspaper the Guardian.

In August 2014, Edelman did not explicitly rule out taking on climate deniers as clients, according to a survey of major public relations firms conducted by the Guardian and the Climate Investigations Center.

But Edelman shifted its stance following a two-year review that concluded that representing coal producers and climate deniers threatened its core business, following negative publicity about its work for the oil lobby and pipeline companies, the Guardian said.

"On climate denial and coal those are where we just said this is absolutely a no-go area," chief executive of Edelman's Europe operations Michael Stewart told the newspaper.

"When you are trying in some way to obfuscate the truth or use misinformation and half-truths, that is what we would consider getting into the work of greenwashing, and that is something we would never propose or work we would support our client doing."

The potential impact of the decision on the global company's finances is unclear. Edelman had $768 million in revenue in 2014 and employs about 5,500 people.