Urgent tests are being carried out Goodmayes Park, Ilford, after the inexplicable death of several swans.
Rescue charity The Swan Sanctuary reported recovering at least three dead swans during the last two weeks.
This is the third incident that has threatened wildlife on the lake since September, when a number of birds died from suspected avian botulism.
Local resident Linda Zussman said she noticed the specialist swan rescue charity recovering unwell animals in the park park on November 15..
She said: “I heard one of the cygnets had died, that was really upsetting.
“It’s part of where I live – that park had been such a blessing during lockdown, – it’s something we should treasure.”
According to Redbridge Council, the rescue service first attended to reports of distressed swans on November 12.
Responders returned five days later and recovered the three swans that had died.
In October, swans had to be removed from the lake and cleaned after engine oil was dumped in the lake. However, it is not thought that this caused the deaths.
A Redbridge Council spokesperson said: “We are very sad and concerned to hear about the three dead swans found in Goodmayes Park on Wednesday.
“We are proud of our parks and take the protection of the wildlife that live in them very seriously.
“In the last two months there have been several unrelated issues concerning the lake and wildlife at Goodmayes Park, all of which we have responded to swiftly with the Swan Sanctuary and in conjunction with Vision RCL, and where appropriate, DEFRA and the Environment Agency.
“We do not know what has caused these birds to die, and the lake is not static, so the problem may have initiated in waters which feed into the lake.”
The spokesperson added the water will be tested several times over the next fortnight to establish a cause of death.
A spokesperson for the Environment Agency said: “The Environment Agency is not involved in any investigations at Goodmayes Park.
“The park is not connected to a water body that is controlled by regulation, or monitored by the Environment Agency.”
Members of the public can report suspected pollution incidents to the Environment Agency’s free 24-hour hotline on 0800 80 70 60.