The wildlife trade monitoring network TRAFFIC - using data from rehabilitation centers as an indicator for the trend - said rapid deforestation poses the biggest danger to the estimated 7,300 Sumatran orangutans left in the wild.
Despite years of legal protection and awareness campaigns, the capture and trade of these apes for pets or local zoos continues to contribute to the decline in these species, said Chris Shepherd, the group's acting director in Southeast Asia.
"Without serious penalties, this illegal trade will continue, and these species will continue to spiral toward extinction," he said, noting that the animals are generally caught when they are young and often handed over to rehabilitation centers when they are too old or big to be held as pets.
More than 140 orangutans were brought to one such center on Sumatra island between 2002-2008 - numbers that mirrored those in the '70s, when the trade went largely unchecked, the British-based group said.
"If this situation continues, the Sumatra orangutan could well face extinction," said Wendy Elliott, species manager at WWF International.
There are an estimated 50,000 to 60,000 orangutans left in the wild, 90 percent of them on the island of Borneo, which is divided largely between Indonesia and Malaysia, and the rest on nearby Sumatra.
Rain forests have been cleared and burned at alarming rates on both islands to make way for lucrative palm oil plantations and for timber.
TRAFFIC also based its findings on visits to wildlife markets and interviews with owners of pet orangutans.
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