The deaths of 20 water birds on a mile-long stretch of river are being investigated by police and environmental groups.

Bodies of 13 swans and seven ducks were found on the River Great Ouse between Mill Lane, Little Paxton and St Neots Marina in just two weeks.

Police announced on Thursday they were investigating how the birds had died, but the Environment Agency (EA) and the Department for Environment, Food and Rural Affairs (DEFRA) have been unable to provide an explanation.

Tests by the EA have shown no signs of pollution in the water and post mortem examinations by DEFRA found no evidence of disease.

Samantha Brown, 25, of Church Street, St Neots, was walking along Brook Street, near the marina, last month and saw four dead ducks.

She said: “I reported it to the RSPCA as I thought it was an environmental health issue and there was rubbish in the brook as well. I’d never seen a dead duck before and we walk along there all the time.”

Christine Green, 53, of Brook Street, had also noticed the dead birds. “There was a woman from the Environment Agency here about a month ago,” she said.

“Somebody had left a huge amount of building rubbish and when she was here she noticed two or three dead ducks and said there were some dead swans further up the river as well.

“She wondered if anglers had been trying to get rid of them.”

An EA spokesman said tests on the water had revealed pollution had not caused the deaths. He added: “If there was a problem with the watercourse we would expect fish to have died. There have been no reports of dead or distressed fish.”

Post mortem examinations by the Animal Health and Veterinary Laboratories Agency also proved inconclusive.

A spokesman said: “We’re doing some more tests but ideally we need birds that have recently been found. If anyone does come across birds in that area that have died or seem unwell they should call the DEFRA helpline on 08459 335577.”

The case has been passed to Natural England and the Wildlife Incident Investigation Scheme to find out whether the birds were poisoned.

Pc Alun Bradshaw said: “It is very unusual for such a large amount of birds to die in such a short space of time and we are keen to find out what caused the deaths.”

Anyone with information should call police on 101.