Almost half of the world's primate species – which include apes, monkeys and lemurs - are threatened with extinction due to the destruction of tropical forests and illegal hunting and trade.

In a report highlighting the 25 most endangered primate species, conservationists have outlined the desperate plight of primates from Madagascar, Africa, Asia and Central and South America, with some populations down to just a few dozen in number.

The golden headed langur, which is found only on the island of Cat Ba in north-eastern Vietnam, is down to 60 to 70 individuals. And there are fewer than 100 northern sportive lemurs left in Madagascar, and around 110 eastern black crested gibbons in north-eastern Vietnam.

Of the world's 634 primate species, 48% are classified as threatened with extinction on the IUCN's "red list" of threatened species. The latest report was compiled by 85 primatologists working in the field and will be launched today at Bristol zoo by a coalition of conservation groups including the IUCN and Conservation International and the International Primatological Society

"All over the world, it's mainly habitat destruction that affects primates the most," said Christoph Schwitzer, head of reseaarch at the Bristol Conservation and Science Foundation and one of the authors of the report. "Illegal logging, fragmentation of forests through fires, hunting is a big issue in several African countries and also now in Madagascar. In Asia one of the main problems is trade in hearts for traditional medicine, mainly into China."

Russell Mittermeier, a primatologist and president of Conservation International, said: "The purpose of our top 25 list is to highlight those that are most at risk, to attract the attention of the public, to stimulate national governments to do more, and especially to find the resources to implement desperately needed conservation measures. In particular, we want to encourage governments to commit to biodiversity conservation measures when they gather in Japan in October. We have the resources to address this crisis, but so far, we have failed to act."

There are fewer than 320 Delacour's Langurs left in Vietnam, thanks to the trade in the animals' bones, organs and other tissues for traditional medicines. The Sumatran orang-utan is down to around 6,600 due to fragmentation of their habitats and the removal of forest to make way for agricultural uses such as palm oil plantations.

Schwitzer said that the primate he monitors, the blue-eyed black lemur in Madagascar, has suffered from the rapid destruction of forests in recent years and now numbers no more than 2,300. "With the political crisis in Madagascar, this has been exaggerated in the last year or two, with lots more illegal logging and now even the government is selling logging concessions more or less illegally."

With some populations of primates down to just a few hundreds or thousands, many species are at imminent risk of extinction. "There are no small disasters for small populations, the disasters are always big," said Schwitzer. "Even if they are a few thousand, they can be wiped out by a couple of events like cyclones. It's very easy for these populations to fail."

Schemes such as the UN's Redd programme (Reducing Emissions from Deforestation and Forest Degradation in Developing Countries), which are being proposed as part of any global deal to tackle climate change, will be crucial in maintaining falling primate popualtions. The idea is that rich countries would pay developing countries to maintain their forests, therefore locking in the carbon and preventing further greenhouse gas emissions. In an enhanced version of the idea, developing countries will be incentivised to plant more trees, growing their forest areas. "Several countries and governments have pledged support - if this is going to happen and it's well done, then we stand a good chance to be able to save some of these primates' forest habitats for the future," said Schwitzer.

And conservation measures can work, if they are put in place in time. There have been some limited successes, for example, in helping some primate species to recover - the black lion tamarin was moved from "critically endangered" to "endangered" on the IUCN red list, similar to the golden lion tamarin in 2003. That was the result of more than 30 years of conservation effort at zoos and by scientists.

Schwitzer hoped the new report would highlight the extent of the dangers facing some of humankinds' closest relatives in the wild. "Support and action to help save these species is vital if we are to avoid losing these wonderful animals forever."