Australia's rarest mammal, the Christmas Island pipistrelle bat, is months away from extinction, and wildlife experts say the government is failing to take action that could save the species.

A recent audit of the pipistrelle – a minuscule bat found only on Christmas Island, an Australian territory in the Indian Ocean – concluded that fewer than 20 individuals remain, all roosting under one piece of bark in the same tree.

Experts say the only way to rescue the species is to capture the surviving bats and breed them in captivity. However, the Environment Minister, Peter Garrett, claims that would entail "unacceptably high risks" to the pipistrelle, which is not much bigger than the tip of a thumb. Instead, he has set up a captive breeding trial involving a closely related micro-bat.

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By the time the trial yields results, the pipistrelle will be extinct, according to Michael Pennay, president of the Australasian Bat Society. Over the past 15 years, the population has fallen by 90 per cent. "If it continues to decline at the same rate, the species will vanish altogether within months," said Mr Pennay, a zoologist.

"It could happen in one day. All it takes is for that tree to fall down – and it's a dead tree."

Australia's leading bat scientists are so concerned that they have drawn up a rescue plan and offered their services for free. A team of experts is willing to travel to Christmas Island, capture the remaining bats and establish them in enclosures at a research station. Mr Garrett has yet to respond.

Australia already has the worst mammal conservation record of any country. Of all the species lost over the past 200 years, nearly half have been Australian. They include the Tasmanian Tiger, or thylacine, which died out in the 1930s. Thanks to conservation efforts, it is half a century since the last Australian mammal, a species of wallaby, became extinct. Now, unless Mr Garrett has a change of heart, the pipistrelle looks likely to join the long list.

Bat experts accuse the government of dragging its feet. The pipistrelle has been classified as critically endangered, the highest risk category, since 2006. Covered in soft fur, the creatures weigh less than a 10p piece. Contrary to popular perception about bats, they have excellent eyesight. They also use a sonar-like system to navigate their surroundings.

Scientists have no idea why the population has decreased so catastrophically, but they theorise that disease, or introduced pests – which include black rats, yellow crazy ants, wolf snakes and giant centipedes – may be to blame. Several other species native to Christmas Island have experienced steep declines, or disappeared.

Most of Christmas Island is a national park, so habitat loss is not a factor. The island was annexed by Britain in the late 19th century; sovereignty was transferred to Australia in 1958, and nowadays about 1,500 people of Chinese, European and Malay ancestry live there.

Mr Pennay insisted that the government trial was unnecessary, since there was already plenty of evidence that micro-bats could thrive in captivity. "The highest priority now is to secure these bats from whatever is threatening them," he said. "If you leave them in the wild, they'll almost certainly go extinct."