Your afternoon chocolate bar may be fuelling climate change, destroying protected forests and threatening elephants, chimpanzees and hippos in West Africa, research suggests.

Well-known brands, such as Mars and Nestle, are buying through global traders cocoa that is grown illegally in dwindling national parks and reserves in Ivory Coast and Ghana, environmental group Mighty Earth said.

"Every consumer of chocolate is a part of either the problem or the solution," Etelle Higonnet, campaign director at Mighty Earth, told the Thomson Reuters Foundation.

"You can choose to buy ethical chocolate. Or you're voting with your dollar for deforestation."