Fareham and Gosport Environmental Health is advising residents do not swim in the lake after two separate sewage outflows discharging into the water.
Gosport Councillor, Lesley Meenaghan, said: “Following reports from members of the public, officers inspected Haslar Lake with Southern Water and confirmed two sewage outflows discharging into the water.
“One outflow appears to stem from an unauthorised connection to a surface-water drain — the source of the second is still under investigation.”
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Water samples have revealed very high ammonia levels in the water, which Cllr Meenaghan saying this indicates long-term pollution.
Effective immediately, residents have been told not bathe, paddle, or swim in Haslar Lake, Workhouse Lake, Stoke Lake, or Alverstoke Creek.
Residents are also advised not to fish or let pets enter these waters.
Warning signs will be in place by tomorrow morning and remain until water quality is fully restored.
Southern Water will tanker away all contaminated water and a temporary bung will be fitted to stop further discharge.
Drainage will then be correctly reconnected to the mains sewer.
A spokesperson for Southern Water said: “We discovered an illegal private connection in which wastewater was flowing straight into a surface water drain from a block of flats, instead of into our sewer for treatment.
“Tankers have been utilised to remove wastewater from the area while we fix this.
“During this work, we also found damage to one of our pipes so are taking this opportunity to repair that too. Sorry for any inconvenience caused while we carry out this activity.”
This comes after Fareham Borough Council confirmed that bathing water from Hill Head to Lee-on-the-Solent had been closed yesterday due to “sewage pollution”.
A sewage pipe had burst in the Salterns Road area of Hill Head and residents were warned not to swim in the area.