But the joy of discovery for the 13-year-old boy who spotted the female mouse-eared bat at an unspecified address in Bognor Regis, Sussex, was short lived. Three days after the animal's rescue, it died.

Disappointed conservationists hope this is not the end, finally, for the rare mammal large enough to fill the palm of a man's hand. They plan to go back in the summer to establish whether others are hanging on in the face of intensive farming and the pesticides that have destroyed so many insects that make up their diet.

The location of last week's brief encounter is being secret for fear of enthusiastic amateurs disturbing other hibernating bats. Amy Coyte, chief executive of the Bat Conservation Society, told how the boy, rushing to feed chickens at the bottom of the garden, made his prized find and alerted his mother. "It was an absolutely freezing day so they brought the bat in and tried to warm it up. The boy was really excited, although it looked really ill and was really thin."

They contacted local volunteer bat workers through a website, and one came to take it home but failed to save its life. "This is a sad but incredible discovery," said Ms Coyte. "The fact the bat was found within five miles of the last known colony suggests it may be a survivor from that or another colony. We know from her worn teeth she was old. She may have lived alone for the last 10 years - but we hope that there are more somewhere in the area."

The last known colony, a few miles from Bognor, was discovered in 1969 and contained several females until 1985 when they all failed to return; it is thought they may have died in a fire in a cottage where they roosted.

There are 16 known species of bat remaining in Britain. Although they are protected by law, two are endangered, nine vulnerable and all threatened.