But now, with fewer than 20 of the creatures remaining and faced with the unwelcome prospect of the first Australian mammal in more than 50 years becoming extinct on his watch, the Environment Minister, Peter Garrett, has finally acted.

However, rather than immediately capturing the remaining bats to start a breeding program, Mr Garrett plans to use a closely related and abundant microbat in the Northern Territory for a three-month test run before tackling the pipistrelle.

He said the Threatened Species Scientific Committee had confirmed extinction of the bat, which weighs less than an empty matchbox, was almost inevitable. "Unfortunately [the committee] also advised me that there is a high risk associated with a proposed captive breeding program," he said.

"The bats are also very difficult to catch and no one knows how to keep them alive for breeding. We will do whatever is practical and feasible to save the pipistrelle. I am deeply concerned by the fact its prospects do not appear bright on the basis of our current understanding of the situation."

Greg Richards of Bat Conservation International, who was part of a recent research trip to the island that found only four bats left in the wild, fears the worst. "The plot of abundance has gone in a straight line downhill since 1995, and statistically that line predicts extinction some time this year.

"It's very frightening for ecologists and conservationists."

Dr Richards was pleased to see action on the pipistrelle but said the latest study would only delay an emergency rescue mission to the island - possibly fatally. What we want to happen immediately is to get a catching team of young fit people with expertise out there to save these bats."

Quite why pipistrelle numbers have plunged by 99 per cent in such a short time remains a mystery to scientists. The finger has been pointed at various pest species, including yellow crazy ants, giant centipedes and wolf snakes, but there is no clear evidence as to the cause.

Dr Richards said one hypothesis even suggested climate change might have a role by delaying the start of the wet season and removing much of the food source for nursing females at the beginning of the year.

"The bat breeding cycle is timed to have babies in December at the start of the wet season when there is an abundance of insects so the huge load on females to produce milk can be accommodated."