By Alex Kirby BBC News Online environment correspondent

A species of bat thought to have died out in the UK a decade ago has been rediscovered by conservationists.

The animal, a young male greater mouse-eared bat, was found hibernating in Sussex, in southern England.

I believe there's possibly a small but significant population of the bats still to find David King, Sussex Bat Group

The bat was found on 14 December during a periodic winter check of known hibernation sites.

Its discovery has raised hope that the species may have managed to survive in the UK.

An elderly female was found in January, 2001, not far from where the latest discovery was made. She died within a few days.

She is thought possibly to have been a survivor of a group of up to 30 greater mouse-eared bats which used to frequent the hibernation site where the young male was found.

Presumed extinct

Before her, the last bat of the species known to have existed in the UK was an animal which wintered at the site from 1975 to 1988.

A greater mouse-eared bat (Image: R.Stebbings/BCT)

Its failure to return the following year led to an official declaration in 1990 that the species was extinct in the UK.

Greater mouse-eared bats declined sharply in north-west Europe during the 1970s and 1980s, though there is some evidence of a recovery since then.

They are one of the largest European bats, with a wing-span of up to 45 centimetres (18 inches). They usually roost in large loft spaces during the summer, and feed mainly on beetles.

Amy Coyte of the Bat Conservation Trust said the latest discovery was "immensely exciting".

She said: "We hope this means that the greater mouse-eared bat is still breeding in the UK, and that this will encourage bat workers to look out for this species."

Sound sleeper

David King is a member of the Sussex Bat Group (SBG) whose members discovered the young male.

He told BBC News Online: "We check well-known hibernation sites during the winter to monitor bat numbers.

"A lot of species use this site, but the greater mouse-eared is the rarest bat we've ever found.

Brown long-eared bat (Image: Hugh Clark/BCT)

"Our members recognised this one straight away, and ringed it, so if we find one next summer we'll know whether it's the same bat or a different one.

"Ringing the bat would have woken it, but it would have settled down again pretty quickly. When the temperature's below 6 Celsius they sleep anyway, because there are no insects around.

Heard but not seen

"We have quite strong hopes now that the greater mouse-eared bat is not extinct here after all.

"I believe there's possibly a small but significant population of the bats still to find. We're all excited about it."

Mr King works for a company, Stag Electronics, which makes bat detectors.

These convert the ultrasonic sounds the bats make in echolocation into audio, allowing them to be identified and their movements monitored although they remain invisible to observers.

Sixteen bat species (apart from the greater mouse-eared) are known to live in the UK. Two are classified as endangered, nine as vulnerable, and all are threatened.