The semi-aquatic mammals, which can grow up to four feet long, were until recently restricted to river banks and waterways in isolated pockets of the countryside. Even there, they were a rare sight.
However, a wildlife survey has found that otters are following species such as foxes and squirrels by adopting an urban lifestyle.
They can be found in town gardens, parks and churchyards around the country, according to the findings of the sixth annual survey of mammals living in towns and cities, conducted by members of the People's Trust for Endangered Species and Mammal Trust UK.
An otter was filmed in the centre of Bristol earlier this year, while there have been reports of the creatures being seen feeding in major cities including Birmingham, Manchester and even in London in the Thames and River Lee.
The Wildlife Trusts estimate that otters are now established in at least 13 towns and cities across the country, and say they have had reports of sightings in 100 other urban locations.
Wildlife experts believe the migration from countryside to town is a result of improving water quality and expanding fish stocks in Britain's rivers. They say that other species are likely to follow.
David Wembridge, who led the Living with Mammals survey, said: "The return of otters to urban waterways is an indicator of the improvement in water quality in Britain's towns and cities. They are probably following the fish populations as they return to rivers.
"Otters can be very bold and hunt in fairly noisy environments. Their presence in the built environment puts conservation firmly in an urban context."
At first glance, towns and cities would seem to be far from an ideal habitat for a shy and solitary species such as the otter. The mainly-nocturnal mammals typically set up their dens in natural river bank hollows or reed beds in marsh lands.
But otters – part of the mustelid family of mammals, which also includes badgers and weasels – have also been known to occupy man-made structures beside rivers if they provide enough shelter and protection.
They need clean water and large quantities of fish to survive, while other successful urban species such as foxes have flourished on the waste that human city-dwellers discard onto the streets.
Otters were widespread throughout the UK until the 1950s, when their populations began to crash, largely because of the use of organochlorine pesticides which poisoned the fish and the otters. By the 1970s, otters had been wiped out in most of England.
A ban on the pesticides and a national clean up of waterways, which has resulted in Britain's rivers and canals being in their cleanest state since the start of the Industrial Revolution, has seen otters return to many parts of the countryside.
The Living with Mammals report, which will be published next month, confirms evidence of otters in eight out of the 700 urban locations surveyed – defined as sites within 200 metres of people's homes – including five urban gardens.
The survey, which is conducted every spring, will also reveal a further fall in urban hedgehog numbers, which have been in steady decline for the past six years.
Mr Wembridge said: "Urban gardens have the potential to be ideal habitats for hedgehogs as they can provide the shelter and invertebrate food they require, but their decline in gardens seems to match the fall in numbers elsewhere in the countryside.
"It is perhaps a reflection of the changes in the way gardens are used and looked after. Paving over gardens removes their opportunity to forage for food and the long grass they prefer is also often attacked by strimmers which can lead to casualties."
Sightings of rats and mice have also increased dramatically in urban areas, perhaps due to last year's mild winter.
Tony Whitbread, from the Wildlife Trusts, said that despite the decline in hedgehogs, the presence of otters in Britain's towns and cities was an encouraging sign.
Otters are known to compete against foreign species such as mink that have been devastating populations of small native mammals such as the water vole, which has declined to a critical level.
He said: "Otters are very adaptive creatures, so they are more than capable of living in urban areas. It is perhaps a reflection of the good interconnected network of waterways that now exist.
"They are such a good barometer species of the health of a habitat, as they need clean water. If water is good enough for otters then it is probably good enough for us to drink. It certainly shows there is hope for other native species."Original here