Controversial aerial surveys aimed at finding oil under Africa's oldest national park have been started by a British company amid fears that drilling in the area could seriously threaten the world's last sanctuary for mountain gorillas.

The moves towards possible oil exploration in Virunga national park, in the Democratic Republic of Congo, have been condemned by the UK government and by the World Wildlife Fund.

This week the WWF is launching a campaign, Draw the Line, against the exploitation of the park, which was established in 1925 and designated a World Heritage Site by Unesco in 1979.

Soco International, whose headquarters are in London, has defended its aerial survey, saying it was being governed and monitored under the terms of the Environmental Acceptability Certificate issued by the DRC's ministry of the environment, nature conservation and tourism.

But earlier this year the Unesco world heritage committee called for the cancellation of all such Virunga oil permits and appealed to two concession holders, Total and Soco International, not to undertake exploration in world heritage sites. Total has since agreed to respect Virunga park's current boundaries, leaving Soco as the only oil and gas company planning to explore inside the park's 7,800 sq km. It claims its area of interest is not near the gorillas' habitat. Rangers and wildlife experts disagree.

Virunga is already in a fragile state, thanks to poachers. In addition, it sits close to the DRC's borders with Uganda and Rwanda and has been affected by influxes of refugees and militias during both the Congo civil war and the Rwandan genocide, as well as ongoing skirmishes with rebel groups. It is home to 200 of the endangered mountain gorillas, a quarter of the world population. Although recent years have been a success story for the park, thanks to the efforts of conservationists and local rangers and the number of mountain gorillas has more than doubled in the past decade, many park staff have been killed by poachers and militias. Virunga is temporarily closed to visitors because of the violence.

Last year the UK government expressed its opposition to drilling inside the park. A Foreign Office spokesman said: "The UK opposes oil exploration within Virunga national park, a world heritage site listed by Unesco as being 'in danger'. We urge any company involved, and the government of DR Congo, to respect the international conventions to which it is a signatory."

Drew McVey is the regional manager for East Africa at WWF in the UK and has just returned from the region. He said: "Virunga has been a fantastic success in the past few years. We've seen the population of gorillas jump and tourists are starting to come to see them. In terms of the local people, they understand the importance of the mountain gorillas to their future prosperity, and we have even had reports of rebel groups in the park no longer poaching, but making money pretending to be authorised tour operators. Ironically that is a sign of how important these big mammals are.

"Virunga has the most biodiversity in all of Africa … it is heavily populated around the park, so there's a massive demand on the park and its resources. The conflict that has gone on in the area adds another dimension of fragility.

"But now to have this terrible threat hanging over it of oil exploration is just so disturbing."