Indonesian coffee growers are illegally clearing an important nature reserve, according to conservation group WWF.
Indonesian coffee farmers mostly grow cheap robusta beans
The Bukit Barisan Selatan reserve, on the southern tip of Sumatra, has already lost nearly 20% of its area to illegal coffee growing, the group says.
The park is one of the few areas where endangered Sumatran tigers, elephants and rhinos still co-exist.
Indonesia is the world's fourth largest coffee exporter. The robusta bean it grows is often used in instant coffee.
"About 17% of the national park area is being cultivated for coffee," Nazir Foead, from WWF Indonesia, told reporters.
"If this trend of clearing park land for coffee isn't halted, the rhinos and tigers will be locally extinct in less than a decade," he said.
The WWF said that the illegally grown coffee was blended with legally produced beans before being sold on to international food and drinks companies.
"WWF determined that most of the companies buying the coffee likely were unaware of its illegal origins," the group said in a statement, adding that it had informed some of the companies involved about the situation.
Mr Foead said the group was also working with farmers to convince them to cultivate coffee outside conservation areas.
"WWF doesn't want to shut down the coffee industry," Mr Foead said.
"We are asking national and multinational coffee companies to help build sustainable coffee production around the National Park, while implementing rigorous chain-of-custody controls that exclude all illegally grown coffee from their supplies."
The report said several well-known brands were involved.
One company, Nestle, issued a statement saying it regretted such unacceptable activities and never willingly purchased coffee from dubious sources.
But the company said it was often difficult to determine the precise origin of its coffee.
The head of the park told the BBC that some 60,000 hectares - around a fifth of the park's total area - had been taken over by illegal plantations, most of them producing coffee.
The park, which covers 300,000 hectares, is policed by only sixty rangers, he said, and around fifty community workers.
Stopping the expansion of the plantations is all but impossible, he said.
The area is home to some sixty tigers and around the same number of rhinos.
Both species are endangered and park officials say destruction of their natural habitat by farmers is making them easier targets for hunters.