For tens of thousands of years, the birds of Gough Island lived unmolested, without predators on a remote outcrop in the south Atlantic.

Today, the British-owned island, described as the home of the most important seabird colony in the world, still hosts 22 breeding species and is a world heritage site.

But as a terrible consequence of the first whalers making landfall there 150 years ago, Gough has become the stage for one of nature's great horror shows. Mice stowed away on the whaling boats jumped ship and have since multiplied to 700,000 or more on an island of about 25 square miles.

What is horrifying ornithologists is that the British house mouse has somehow evolved, growing to up to three times the size of ordinary domestic house mice, and instead of surviving on a diet of insects and seeds, has adapted itself to become a carnivore, eating albatross, petrel and shearwater chicks alive in their nests. They are now believed to be the largest mice in the world. Yesterday Birdlife International, a global alliance of conservation groups, recognised that the mice, who are without predators themselves, are out of control and threatening to make extinct several of the world's rarest bird species.

The organisation, which runs the Red List of endangered bird species, elevated the Tristan albatross, of which only a few remain in the world, and the Gough bunting, a small finch found only on the island, to the list of the world's most critically endangered species, the highest category of threat. Five other bird species on the island are also said to be threatened.

The RSPB has proposed hiring helicopters to drop thousands of tonnes of rodent poison on the of the volcanic island 2,000 miles off the coast of south America. "A government-funded feasibility study done with New Zealand, which has eradicated rats from many islands, shows it is possible. The mice would take the poison and just go to their nests and die. We think it could be done fairly easily and would cost about £2.6m," a spokeswoman said.

"Things are getting worse on Gough," said Dr Geoff Hilton, an RSPB scientist who has been researching conservation problems in UK overseas territories. "In the presence of house mice, the albatross and bunting have no chance of survival. The only hope for these threatened birds is complete eradication of mice.

"The world's greatest seabird island is being eaten alive, as the mice are likely to be affecting the fortunes of many seabirds on the island. Without help Gough Island will be likely to lose the majority of seabirds," said Hilton.

Those who have witnessed the phenomenon say the mice attack at night either alone or in groups, gnawing through the nests to get at the baby birds. Their parents, who have never experienced predators, are unable to defend them.

Studies suggest about 60% of all Gough's chicks die in their nests. "It is a catastrophe. The albatross chicks weigh 10 kilograms. They evolved on Gough because it had no mammal predators - that is why they are so vulnerable. The mice weigh 35 grammes; it is like a tabby cat attacking a hippopotamus," said Hilton.

Britain has long been criticised for not maintaining the ecology of its overseas territories, which are mainly made up of groups of islands such as Pitcairn, Tristan da Cunha, and the Falklands. Of the world's 190 most endangered birds, 32 are now officially British responsibility.

The RSPB's spokeswoman added: "The study shows there is a glimmer of light. The UK government has supported us in discovering the problem, in conducting the feasibility study, and now in finalising our plan for the mouse eradication.

"The big question is whether the UK will take its international commitments seriously and do what the governments of New Zealand and Australia have done, and provide the big money needed to actually do the mouse eradication.

"If they don't, we won't be able to give two critically threatened species the lifeline they need."

The discovery that the mice had adapted their diets and supersized themselves was made by Richard Cuthbert, a professional ornithologist who spent a year on the island in 2001 and stumbled on the phenomenon as he was leaving.

"It sounds incredulous, implausible that a mouse could attack a chick, but these chicks are really big spherical balls of fat covered in down, and because they are so fat and big they cannot defend themselves," he said.