Environmentalists have reported a dramatic increase in the number of otters being killed on roads - damaging efforts to boost otter numbers in rivers across England and Wales.

In the past three months 34 otters were reported to Water UK and the Wildlife Trusts Otters and Rivers Project as having being killed on roads in England and Wales, double the number in the previous three months, and more are being reported each week.

Most of the reports are from Cornwall, Devon, Somerset, parts of Wales, Northumberland and the River Severn catchment.

Over the past 20 years the number of adult male otters killed in road traffic accidents in Wales has increased fivefold, with 40 animals found in 1998 compared with an average of eight between 1980 and 1989.

Eight otters were killed on roads near the River Axe, on the Devon and Somerset border, in the past three months - nearly a fifth of the estimated otter population on the river.

Male otters appear to come off worst in the survey, probably because population expansion forces young adult males to seek new territories. They often see roads as an easier alternative to swimming against strong currents, particularly in flood-swollen rivers.

Female otters are less likely to travel widely and encounter roads, although in Northumberland, where the otter population is high, females are increasingly being killed.

Lisa Schneidau, director of the Water UK and the Wildlife Trusts Otters and Rivers Project, said: "As otters are starting to move back into areas where they have not lived for 30 years, road deaths are a serious threat to their re-establishment. The recent increase in road deaths is alarming."

She called for tunnels and ledges to be included in road schemes, to protect the animals. The Highways Agency has already published guidelines for local authorities.

The Otters and Rivers Project has a network of 24 conservation officers based at Wildlife Trusts in England and Wales, working with the Environment Agency.