The shy and endangered mammals have not been spotted on some waterways for decades.

Sightings suggest that dog otters from recently restored habitats on the Upper Severn and River Wye in Wales are now venturing into border regions and further into middle England.

Conservationists tracking the creatures have found them in Stratford-upon-Avon, Warwickshire, and just five miles outside Oxford, on the Thames.

From north-east Wales, regular sightings have been made down the river Dee from Llangollen towards the outskirts of Chester.

'Otters need space'

Geoff Liles, director of the Otters in Wales project, said: "Otters need plenty of space and a male can range over 40km in search of food and mates.

Otters venturing further

"As the population builds up in Wales the evidence is that more are now roaming across the border into England to re-colonise areas that haven't seen otters for many years.

"It's really quite an upbeat message into the year 2000, but there's still a long way to go."

The Government's UK Biodiversity Action Plan aims to restore breeding otters by 2010 to every watercourse and coastal area where they have been recorded since 1960.

Water companies, the Environment Agency and Wildlife Trusts have joined together in a resue operation to enhance the otters' habitats, particularly on rivers in urban areas of England.

The strongest populations remain in Wales, Scotland, parts of Devon and Cornwall and in East Anglia.