Thames Water is investigating ongoing pollution issues at Harrow Lodge Park and the River Ravensbourne.
Sightings of dead fish have been reported at the Hornchurch park lake for years, with Havering Friends of the Earth (Havering FoE) co-ordinator Ian Pirie saying the group discussed it with the council back in 2017.
The most recent instance was this summer, with a Havering Council spokesperson telling the Recorder that misconnections – when pipes are incorrectly attached to a surface water drain - “likely played a part”.
Thames Water, which manages the local sewage system, disputed this.
A spokesperson said it had found “no suggestion of foul water entering the watercourse at any point”.
Instead, they said the recent extreme heatwave had resulted in an oxygen crash within the water, killing the fish.
Since then, Mr Pirie organised for London waterkeeper Theo Thomas to visit the site and assess the potential issues involved.
During that visit on October 12, Mr Thomas says he identified two problems.
He claims water was flowing into the River Ravensbourne, and subsequently into Harrow Lodge Park, from a water culvert which mainly takes rainwater into the river.
However, with little recent rain and the alleged presence of toilet paper fragments, he believes this indicates misconnections from nearby housing have indeed been polluting the river.
Mr Thomas also allegedly found several manhole covers running alongside the river showing signs of overflow, again involving disintegrated toilet paper.
A build-up of silt is one potential cause suggested.
On his findings, Mr Thomas said: “People who care about Harrow Lodge Park and the River Ravensbourne are right to be worried.
"There are clear signs the local sewer system can’t cope. Dumping sewage in the park is illegal and a risk to public health.
“We’ve asked Thames Water to check the condition of the main pipe that runs under the park and take action.”
Mr Pirie added: “This unacceptable pollution is almost certainly one of the factors (along with the hot weather and a build-up of silt) that led to the recent mass die-off of fish in the lake.
“Whether it was a significant factor or not, it is not right that sewage should be flowing into a river in a public park.”
A Thames Water spokesperson said all previously identified misconnections had been resolved, although noted they can “can happen at any time” at customer properties and businesses.
They added Thames Water has "arranged to investigate the issues reported at the River Ravensbourne and Harrow Lodge Park”.
Havering Council has previously said it is carrying out a series of improvement works following the appearance of the dead fish earlier this year, including clearing out the silt lagoon and eastern lake, as well as monitoring the oxygen levels.
The Environment Agency was also approached for comment.
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