Around half of the world's primate species are facing extinction, researchers said.
Some 300 of the 634 different types of ape and monkey on Earth could disappear if no action is taken to save them, according to the latest Red List of Threatened Species.
The list identifies destruction of the rainforests where primates live as the biggest threat to their existence. Other major threats include hunting for food and the illegal wildlife trade.
Russell Mittermeier, from the International Union for the Conservation of Nature (IUCN), which carried out the survey, said: "Tropical forest destruction has always been the main cause, but now it appears that hunting is just as serious a threat in some areas, even where the habitat is still quite intact.
"In many places, primates are quite literally being eaten to extinction."
The survey found that 70% of primates in Asia were in some way threatened with extinction. In Vietnam and Cambodia 90% of primate species are considered at risk, in part because they are hunted for ingredients for Chinese medicine.
Jean-Christophe Vie, deputy head of the IUCN Species Programme, said the situation in south-east Asia was "terrifying".
"To have a group of animals under such a high level of threat is, quite frankly, unlike anything we have recorded among any other group of species to date."
In Africa, the red colobus monkey was found to be particularly at risk, with 11 of the 13 different kinds listed as critically endangered or endangered. Two types may already be extinct, the researchers warn. The Bouvier's red colobus has not been seen for 25 years, and the Miss Waldron red colobus for 30 years.
The findings form part of a survey of all the planet's mammals, which will be published in October.
