Residents and tourists are being warned to stay clear of the village pond at Falmer until further notice.
Test results show the popular pond, beside St Lawrence’s Church, is contaminated with blue-green algae which can be very harmful to the public and animals.
Brighton and Hove City Council has erected warning signs and is working closely with the Environment Agency to resolve the situation, while contacting the public.
Cllr Gill Mitchell, chair of the city’s environment, transport and sustainability committee, said: “It’s extremely important people are aware of this blue-green algae and the danger it poses to themselves and their pets.
“Many dog walkers allow their pets to run into the pond, but we would urge them to stay clear until the signage comes down and we say the pond is safe again.”
Although algae occurs naturally in inland waters, one of the main reasons for algae developing in the pond is visitors feeding the birds and fish. This maintains high levels of nutrients in the pond which can result in algae blooms.
The blooms produce toxins that can kill wild animals, livestock and pets. They can also harm people, producing rashes after skin contact and illnesses if swallowed.
The algae also uses up oxygen in the water at night which can suffocate fish and other creatures.
Although the pond is situated in Lewes District, the land is owned by the city council which is responsible for its upkeep.
Cllr Mitchell added: “The Environment Agency alerted us to this danger on Wednesday (March 21) and our teams immediately began taking action to warn people.
“We have erected signs and will be working with the Environment Agency on the future management of the pond to ensure it is safe in the future, though this may take some months.”