British otters are healthy and reproducing across the country, a report says today.

Announcing the results of the most wide-ranging otter study ever carried out in Europe, the Environment Agency said that lower levels of pesticides have given the nation’s otters a new lease of life.

The agency’s Science Department carried out a series of studies into the deaths of nearly one thousand otters between 1992 and 2003. It considered the factors affecting their survival, the effects of chemicals in rivers and the overall health of the creatures.

The researchers found that otter numbers have begun to expand again after a decline between 1960 and 1980 because of organochlorine chemicals, such as the insecticides dieldrin and aldrin, in the environment.

Lyn Jenkins, an agency conservation officer who managed the study in southern and southwest England, said that dieldrin had been progressively withdrawn from use from 1962 and banned by 1989.

She said: “This was a very potent insecticide, used extensively by farmers as it remained active for a long time after they applied it. But we now know it can take up to 25 years for 95 per cent of dieldrin in soil to disappear. This persistence and the fact that it passes from animal to animal through the food chain was the reason it was eventually banned.

“By interfering with vitamin A levels, it caused reproductive abnormalities and other conditions. Research has strongly linked its use to declines in predators such as peregrine falcon, kestrel and heron.

“Otter numbers dropped significantly during the late 1950s when dieldrin and aldrin came into use, and it seems otters in this country are only just recovering from the effects.”

The greatest danger facing otters was now the car, the survey found. Starvation and other otters were other main causes of death.

Ms Jenkins said there were no population estimates for otters in Britain, but it was unlikely that the number of otters killed during the past 15 years was having a significant impact on the population as a whole.

She said: “It’s also promising that an otter was found on the River Thames last year not far from Tower Bridge – it was the first wild otter to be seen in Central London for more than a hundred years.”

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