The Eastern Gorilla, the largest primate on the planet, is classified as critically endangered after illegal hunting decimated numbers in recent decades, say the IUCN. The population of Eastern Gorillas is now thought to be below 5,000 individuals, a 70% decline in the last 20 years.

With the new classification, four out of the six great apes are now critically endangered – the Eastern Gorilla joining the Western Gorilla, Bornean Orangutan and Sumatran Orangutan as "one step away from extinction."

The Eastern Gorilla has two subspecies which have fared differently. The Mountain Gorilla subspecies has actually increased in population to around 880, while Grauer's Gorilla, the other subspecies, has lost 77% of its population in the last two decades.

"To see the Eastern gorilla – one of our closest cousins – slide towards extinction is truly distressing," said IUCN Director General, Inger Andersen. "We live in a time of tremendous change and each IUCN Red List update makes us realise just how quickly the global extinction crisis is escalating. Conservation action does work and we have increasing evidence of it. It is our responsibility to enhance our efforts to turn the tide and protect the future of our planet."

Evidence of the effects conversation efforts are having may be found with the Giant Panda, which was reclassified as Vulnerable after years of being an icon for endangered species.

The new classification of Eastern Gorillas follows a report from the Wildlife Conversation Society (WCS) and Fauna & Flora International which showed Grauer's Gorilla's incredible decline. The WCS also noted that there are few Grauer's Gorillas in captivity so if it were to become extinct in the wild it would be near impossible to save.

WCS say the main cause of the decline his hunting for bushmeat, often by camps established by armed groups in the east of the Democratic Republic of Congo. Executive Director of the WCS Africa Program, Tim Tear, said the report is a "wake-up call" and "demands more investment to support conservation in the field if we are to save this species."