Photo: NATIONAL PICTURES
There are fewer than 300 of the sub-species left in its entire range in Cameroon and Nigeria and it is classified as critically endangered.
The film was taken by a crew who staked out a stand of fig trees in the Kagwene sanctuary in Cameroon for weeks.
"Eventually, we identified and staked out some of the gorillas' favourite fig trees, which is where we finally achieved our goal.Dr Roger Fotso, director of the Wildlife Conservation Society's (WCS) Cameroon programme, said: "These gorillas are extremely wary of humans and very difficult to photograph or film.
"It's unbelievable that one great ape subspecies has never been filmed."
Due to the steep mountain terrain, tracking gorillas in Kagwene is time-consuming and sometimes treacherous.
Through the years, WCS researchers have developed an effective non-invasive monitoring system aimed at keeping track of the gorillas without disturbing them or exposing them to humans.
After several weeks, the crew finally filmed several minutes of two gorillas feeding on figs some 30-40 feet above the forest floor.
Dr James Deutsch, WCS Africa director, said: "These extraordinary images are vital for the fight to save the world's least known and rarest ape, as well as the mountain rainforest on which they depend."
While many populations of gorillas are threatened by poachers, the gorillas of Kagwene have been protected by the local belief that the apes are people and therefore cannot be hunted or consumed.
Elsewhere, hunting continues to be one of the biggest threats to Cross River gorillas in addition to habitat destruction.
Gorillas are occasionally targeted by bushmeat hunters in the region, and genetic analysis of the population reveals a reduction in numbers over the last 200 years that is most likely due to hunting.
The fragmentation of their forest habitat is caused by farming, road-building, and the burning of forests by pastoralists to encourage new grass for their herds.
It is one of two subspecies of western gorilla, the other being the western lowland gorilla. The eastern gorilla includes two subspecies: the eastern lowland gorilla, and the famous mountain gorillas of the Virunga Mountains and southern Uganda (the latter of which numbers approximately 700 individuals).
The only previous footage available of the rare apes was taken from a long distance with a shaky, hand-held camera in 2005 by a field researcher.