Specialist vets and experts from Doncaster Council have been called in to determine how 100 ducks at a popular Doncaster park have died in the last few days.
The lake at Sandall Park has been sealed off and people are being urged to stay away from the water while further investigations are carried out.
Environment Agency officials visited the park in Wheatley yesterday to carry out tests on the water.
A spokesman said: "Our investigation has shown that there is no pollution in the water in the lake at Sandall Park.
"We have referred this incident on to the Animal Health and Veterinary Laboratories and Doncaster Council so they can look into it further."
Peter Dale, Director for Regeneration and Environment at Doncaster Council said: “We are aware of the death of ducks in Sandall Park and we are liaising with the Animal and Plant Health Agency. We are awaiting further information from them.”
The alarm was raised late on Monday after anglers found the bodies of dozens of dead ducks and coots floating in the lake - although the problem does not appear to have affected the park's fish.
Since then, the lake has been cordoned off and signs erected urging people to stay away from the lake, although the park itself remains open.
Parents are being urged to keep children away from the water and dog walkers are also being urged to keep pets on leads while further tests are carried out.
A spokesman for Friends of Sandall Park, the volunteer group which looks after the park, said: "The Environment Agency have issued an order and nothing is to be moved or put in the lake; no-one is to go near the lake.
"Vets are on their way from York. Dogs are to be kept out of the lake and on a lead near it."