The study - which appears in the July issue of science journal Oryx - discovered the population on Malaysia's Borneo island fell by 10 per cent to 49,600 apes.
"It's disappointing that there are still declines even though there have been quite a lot of conservation efforts over the past 30 years," Wich said.
Researchers said that the orangutan losses on Borneo were occurring at an "alarming rate" and described the situation on Sumatra as in "rapid decline".
"Unless extraordinary efforts are made soon, it could become the first great ape species to go extinct," they said.
The study is the latest in a long line of research that has predicted the demise of orangutans, which are only found in Indonesia and Malaysia.
In May, the Center for Orangutan Protection said just 20,000 of the endangered primates remain in the tropical jungle of central Kalimantan on Borneo island, down from 31,300 in 2004.
Based on that estimate, the study concluded that orangutans there could be extinct by 2011.
Michelle Desilets, director of the Borneo Orangutan Survival Foundation in the UK, praised the new study as a comprehensive look at the orangutan population.
"What matters is that the rate of decline is increasing, and unless something is done, the wild orangutan is on a quick spiral towards extinction, whether in two years, five years or 10 years," she said.
Desilets was not involved in the research.
Indonesia and Malaysia, the world's top palm oil producers, have pushed to expand plantations amid a rising demand for biofuels, which are considered cleaner and cheaper than petrol.
But Wich's study notes there is room for "cautious optimism".
Susilo Bambang Yudhoyono, the Indonesian president, announced a major initiative to save the nation's orangutans at a UN climate conference last year, and the Aceh governor has declared a moratorium on logging.
Many also hope that Indonesia will protect millions of acres of forest as part of any UN climate agreement that will go into effect in 2012.