It has been a long and perilous journey, but otters have finally managed to swim back from the brink of extinction and into every county in England.

Two otters have been spotted building their holts on the banks of the rivers Medway and Eden in Kent, delighting conservationists who had previously predicted they would not return to the county for another 10 years.

"The fact that otters are now returning to Kent is the final piece in the jigsaw for otter recovery in England and is a symbol of great success for everybody involved in otter conservation," said Alastair Driver, the national conservation manager for the Environment Agency.

Otters have reappeared in places where they have not been seen since the industrial revolution, including Bristol, Birmingham and Manchester, and even on the Thames and the Lea in north London. A recent survey on the river Ribble, in Lancashire, showed a 44% increase in otter numbers since 2008.

The Kentish otters herald a remarkable – if slow – renaissance for the sleek, fish-devouring member of the mustelid family, which declined by 95% of its range in western Europe during the 20th century.

In England the otter disappeared dramatically between the 1950s and 1970s because of persecution and pesticides washing into waterways.

After otter hunting was belatedly banned in Britain in 1978, numbers began to increase – particularly following the withdrawal of organochlorine chemicals and a more general improvement in water quality, leading to more fish in rivers and lakes.

The resurgence of the otter, which is top of the food chain in river environments, is an indicator that English rivers are at their healthiest for more than 20 years, according to the Environment Agency.

Terry Nutkins, the naturalist and friend of Gavin Maxwell, author of Ring of Bright Water, said he was "absolutely overjoyed" by the return of the otter across England.

"They are such a beautiful species of the weasel family and part of our heritage," he said. "It's good news and shows that the rivers are clean and there are more people becoming involved with environmental issues."

A spokesperson for The Wildlife Trusts said: "This is fantastic news. We will continue our work to improve habitats for these magnificent animals and to promote the otter as a flagship species of healthy wetland ecosystems. However, we must not be complacent. There is still a great deal of work to do before otters are widespread once more."

The resurgence of the otter has not delighted everyone, however, and anglers have reported otters decimating stocks in fishing lakes. The angler John Wilson recently called the otter "a wanton killer" and some fishing groups have called for a cull. Many angling clubs have been forced to erect expensive fences around lakes to keep otters out.

Some conservationists warn that sightings of otters in new habitats may reflect otters roaming more widely in search of food rather than a big increase in numbers.

Grace Yoxon of the International Otter Survival Fund said evidence of a surge in otter numbers should be treated with caution. "We just don't have the data [on population increases]," she said.

Otters are slow to reproduce and most mothers only bear two sets of cubs in their lifetime.

"It's not physically possible for them to spread very quickly," said Yoxon. "The biggest problem is human encroachment and the destruction of habitat, and increasingly many otters are hit on the roads."

Otters are also sometimes caught in crayfish traps.

The Environment Agency, working with partners including wildlife and angling organisations, has this year been granted an additional £18m of funding by the Department for Environment, Food and Rural Affairs to help more English rivers meet new EU targets on the health of rivers.

Otters have also benefited from reductions in the volume of water extracted from rivers by water companies, farmers and industry. According to the Environment Agency, around 35m fewer litres a day are now being taken from the River Darent in Kent than 20 years ago, support larger populations of wildlife including brown trout and pike.