Bank holiday visitors to north Tipperary's Lough Derg shoreline are being advised not to bathe in the lake because of the presence of blue-green algae.
Last week North Tipperary County Council posted warning signs on the lake's shoreline resorts. They remain in place.
The council has received many calls from people travelling from Dublin and elsewhere for the weekend about whether bathing and water sports are permitted in the lake.
The council is advising people not to bathe in the lake. The algal bloom's toxicity has, to date, tested negative, but the council warns this could change within hours.
"It [blue-reen algae] is negative at the moment. But the situation could change overnight," said Marcus O'Connor, senior environmental engineer with the council.
"We are advising people not to swim or do other water sports on the lake. The warning will remain in place over the weekend," he said. People are advised to keep livestock and animals away from the lake. Drinking water in Nenagh, which is drawn from the lake, remains at a satisfactory standard but it is being constantly monitored.
