Gorillas are being dangerously stressed by tourists whose attentions are disrupting the animals' feeding routines and making them aggressive. The discovery – made by researchers who have just completed a year-long study of the great apes at Bai Hokou in the Central African Republic – has important implications for the tourism industry.

Eco-tourism has become extremely popular, providing travellers with opportunities to get close to rare species, including tigers, polar bears and gorillas. Money raised in this way has helped to preserve endangered animals and bring employment to developing countries.

Gorilla tourism in particular has boomed, providing jobs and business opportunities in several African countries. In Uganda, gorilla tourism brings in an estimated £345,000 a month from the sale of permit fees alone.

But now scientists warn that greater care will be needed. Not only do tourists disturb the animals, but so do research teams studying the animals' behaviour and their interaction with tourists. It is recommended that the minimum distance between humans and gorillas be increased from seven to 18 metres.

"We got a lot of warning barks from the male silverback in a band of gorillas if we went too close," said Michelle Klailova of Stirling University. "And you ignore a male gorilla's barking at your peril, for there is a real danger that it will turn into something much worse, like a full-blown charge. They can kill very easily. They know exactly where to bite a person."

Great apes are known to be vulnerable to human diseases. A common cold has the potential to kill an entire family group. Research at the Tai chimpanzee project in Ivory Coast found that 15 young chimpanzees who died in three disease outbreaks there had been infected with viral strains that were very similar to those found in humans.

Authorities have so far insisted that humans remain at least seven metres away. "Even if we sneeze, then particles from our breath will not reach the gorillas," said Klailova, whose study – written with Chloe Hodgkinson and Phyllis Lee – is published in the American Journal of Primatology. "This barrier has been created for physiological reasons. However, our work suggests that there are good reasons to establish a far bigger gap between humans and animals – for psychological reasons."

Klailova and her research group concentrated on one silverback named Makumba and recorded his vocalisations, daily activities and interactions with his 12 family members. They then studied how these types of behaviour changed when different groups of humans – which included local trackers, scientists and often tourists – came close.

As numbers in a group increased, the gorillas spent less time feeding and instead behaved in a disturbed, unfocused manner. Klailova found that Makumba was more likely to stop feeding and start watching humans as observers moved closer.

Making a male gorilla angrier could lead to him attacking humans or female gorillas in his own band. Either way, the reaction reveals that animals that are now hovering at the edge of extinction are being further stressed.

Klailova admitted that a recommended gap of 18 metres "is not a realistic goal in dense forests, particularly for tourists who have spent valuable time and effort to see the gorillas". But in clearings and in open land, where there are good sight lines, it should definitely be adhered to.

The suggestion will find support from other scientists who have called for increased protection for great apes. Other proposals have included suggestions that all tourists be required to wear face masks to block any transmission of human diseases.

However, the prospect of wearing masks all the time while only being allowed distant glimpses of animals could have a detrimental impact on gorilla-watching holidays.

Eco-tourism has become an important source of income for remote African communities. Foreign visitors who come to enjoy the sights and sounds of natural parks have become a strong motivation for governments to invest in conservation, while the presence of researchers, tourists and tourism infrastructure can work as a strong deterrent to poachers.

As Klailova has said: "It is a very delicate issue."