The well known lake and cockle pond has been fenced off after an unknown bacteria was found in the water.
The boating lake and pond at Walpole Park, Gosport, was cordoned off mid September.
The leader of Gosport Borough Council said tests will be carried out on the bacteria to see whether it could be harmful to humans.
Cllr Mark Hook said: "Tests showed the bacteria has shot through the roof with some extraordinary high readings.
It now needs to be tested to see if it is a harmful or non-harmful one. We have fenced the area off as a precaution at this stage".
The water is checked regularly along with local beach and creek waters by environmental health officers and has been fine up until the most recent tests.
In years gone by the council drained and cleaned the pond annually, reducing the problems of any likely bacteria. But since the pond was made a site of special scientific interest we have been unable to do this. The problem has been made worse by the large number of swans and geese which use the pond. Sadly despite notices asking the public not to feed the swans, many people still do, resulting in more swans attracted to the food source and more pollution of the water.
The council is currently in talks with Natural England and the Environment Agency to see what can be done to solve the problem.