DOG owners are being warned not to let their pets swim in Doffcocker Lodge after several birds died because of a mystery substance in the water.

A number of Canada geese, coots and ducks are reported to have died after spending time in the water, and the Lancashire Wildlife Trust (LWT) confirmed that a dog was taken ill on Thursday following a swim in the lake.

Bolton Council has put up warning signs around the lodge after the substance, first thought to be poisonous blue-green algae, claimed the lives of the birds.

Tests by the Environment Agency came back negative for the algae.

A spokesman for the LWT said: “We have received some calls from members of the public linked to death of wildfowl.

“It is a Bolton Council owned site, so we are in the process liaising with them to support them with efforts to minimise the risk of harm to visitors to the nature reserve and identify the cause of death for species at the site.”

The nature reserve, which is managed by the LWT, is home to common terns and great crested grebes, as well as birds of prey such as sparrowhawks, kestrels and buzzards.

The lodge, located on the western outskirts of Bolton off Moss Bank Way, is a popular spot for dog walkers. A council spokesman said: “We were alerted to issues earlier in the week and consulted the Environment Agency, which is carrying out tests for suspected blue green algae.

“We have placed warning signs at the lodge to inform dog walkers and other visitors to be aware of the potential dangers, and they will be in place until the algae clears up.”

A spokesman for the Environment Agency said: “A sample of the water was taken on Tuesday, but the test for blue-green algae came back negative.

“However, as more sick wildfowl have been found we would advise visitors to still adhere to the warnings until we carry out a full test next week.”