Otters in Britain, which only 30 years ago were thought to be on the brink of extinction, have made a remarkable comeback and are now to be found almost everywhere, according to a report published today by the Environment Agency. Where once the population may have been as low as a few hundred, it is now thought to run into thousands.

In the 1970s, the animal had retreated to Northern Ireland, Scotland, Wales and the far south-west of England, but the report says they are now present everywhere except Kent – and they are expected to return to rivers there too within a decade.

The otter resurgence has been so successful that the River Wye and some rivers in the south-west have now reached their maximum capacity. Otters are also widespread in Cumbria and Northumbria, Wessex and the upper reaches of the Severn.

The otter's return is attributed to the ban on organo-chlorine pesticides in the 1970s and its designation as a protected species. Improved river water quality has also brought back greater numbers of fish such as salmon.

The species is a much cherished member of the British wildlife population, but elusive and rarely seen. It is still best known through literature, from the pages of Henry Williamson's 1920s' novel Tarka the Otter, or for its fleeting appearances in Wind in the Willows - "a swirl of water and a 'cloop!' and the may-fly was visible no more (and) neither was the otter," - but now bolder members of the species can even occasionally be seen around town.

Andrew Crawford, who compiled the report, said: "The numbers must be

back up into the thousands by now. They have not yet been spotted around Westminster Bridge yet, as they were in the 19th Century, but I live in hope that it will happen again within the next few years."

Now one of the chief threats to otters is being run over as they cross roads between streams. In some areas, special culverts, tunnels and runs have been built. The Eurasian otter, lutra lutra, can live for up to 10 years and produce an average of two babies per litter. Young stay with their parents for about 18 months and become fully mature in two years.

Paul Raven, head of conservation and ecology at the Environment Agency, said: "The otter is at the top of the food chain, and as such is an important indicator of the health of English rivers. Their recovery from near-extinction shows how far we've come in controlling pollution and improving water quality. Rivers in England are the healthiest for over 20 years, and otters, salmon and other wildlife are returning to many rivers for the first time since the industrial revolution. We will continue to work with farmers, businesses and water companies to reduce pollution and improve water quality even further, to ensure the full recovery of the otter across the country."

The agency's fifth otter survey in England examined more than 3,000 river sites between July last year and March 2010. Likely sites are searched for signs of otter presence such as paw prints or spraint - droppings. It found more than half bore signs of the animals, up from a third eight years ago and a ten-fold increase on 30 years ago. East Anglia and the Thames have shown the greatest rate of growth since the last survey in 2002.

The rural affairs minister Richard Benyon said: "Thanks to dramatic improvements in water quality we have seen the return of the otter to every region in England, bringing it back from the brink of extinction. Although our rivers are the healthiest they have been for some time, there is always more that can be done to improve water quality and help wildlife return to our rivers. We will continue to work together to deal with pollution problems where they exist in a way that is effective."

Crawford said: "Whether you will see an otter is totally down to luck, but there is a better chance now than there has been for 50 years."