The blue-green substance has been spotted in the waters at both Hartsholme Country Park and Swanholme Lakes Local Nature Reserve in Lincoln.

Notices being installed at the lakeside warn the algae can be fatal to animals and ask people to stay away.

Officers from the City of Lincoln Council say there is no danger as long as the public cooperate.

Known scientifically as cyanobacteria, the organisms can cause rashes, sickness and vomiting for people who touch or swallow them.

In a statement, the council said: "It is important to note that the algae poses no threat to health whatsoever provided individuals and animals do not come into physical contact with it.

"Although there is no general risk to park-users or local residents, the public is being asked to co-operate by keeping away from the lakes."