The lake at Sandall Park has been shut to anglers and dog walkers are also being advised to stay away from the lake after 'a large number' of dead birds were found at the popular beauty spot in the last few days.
Scott Holland, chairman of Sandall Park Angling Club, said that more than 100 dead ducks had been found around the banks of the lake and that Environment Agency officials and vets are due to visit the park this morning to investigate the problem.
He said: "We are advising people to stay away from the lake as a precaution. We're putting signs and tape up and sealing it off.
"We don't want people going near the water or feeding the ducks or going anywhere near them until we know what we are dealing with."
Mr Holland said the outbreak was at the moment only confined to the park's ducks and coots and that geese and fish in the lake had so far been unaffected.
He added: "We are talking about almost the entire population of ducks at the park dying in a matter of days. We think there's only about five or six left alive."
"It is distressing for people so we just want them to stay away while it is dealt with."
Sandra Crabtree, chairman of Friends of Sandall Park, the volunteer organisation which looks after the park said: "It is believed to be something that is only affecting the birds and not the fish that is killing them.
"Please don't feed the ducks or geese or go near them.
"The Environment Agency have been out and told the anglers not to do any fishing and the lake has been closed to anglers.
"It might be advisable to keep dogs on a lead in case they go in the lake to cool off, or find a dead duck.
"The Angling Club are to be congratulated on noticing the problem and taking immediate action."