The study was presented at a conference of The Pan African Sanctuaries Alliance (PASA) in Johannesburg. PASA sanctuaries care for orphaned or injured great apes.

"It is believed that the illegal hunting and eating of apes - known as the bushmeat crisis - has had the greatest impact on the rate of decline, along with deforestation, human encroachment and disease," PASA said in a statement.

"The situation is much more critical than we thought," said Norm Rosen, an anthropologist at California State University-Fullerton who coordinated the study.

The study used the rate of orphans brought by people to sanctuaries to calculate the loss of chimpanzees in the wild - and showed a dramatic increase in the number of baby chimps losing their parents.

Rosen's study - which estimates that 10 chimpanzees in the wild are killed for every orphan that reaches a sanctuary - predicts that the vellerosus subspecies will become extinct in the next 17-23 years.

The other three chimpanzee subspecies face slightly better odds, but all are expected to disappear in 41-53 years, at current rates of decline.

"The numbers at the sanctuaries don't lie. You don't get the kind of steady stream of orphaned chimpanzees we're seeing without a devastating drop in the wild population," said Rosen.

Chimpanzees are found in western, central and eastern Africa.

The 19 PASA sanctuaries currently care for approximately 670 chimpanzees, a number that has risen by more than 50 percent in the last three years.

The study is the latest to sound the alarm about the fate of the great apes, which consist of chimps, gorillas, bonobos and the orangutans of Asia.

One recent UN study said less than 10 percent of the forest home of Africa's great apes will be left relatively undisturbed by 2030 if road building, construction of mining camps and other infrastructure developments continue at current levels.