A water quality expert has warned a park has become “a sewer” and called for urgent action after waste seeped out of manholes.

Theo Thomas, from the charity London Waterkeeper, visited Harrow Lodge Park, Hornchurch, last week following reports of hundreds of dead fish in the lake in July.

Speaking after the visit, he said: “There is sewage rubbish stuck on the bars of the lake’s drain and sewage fungus in the water.

“There is toilet paper, wet wipes and toiletry waste on the ground and on the steps going down to the lake. I also spotted some cotton buds around manholes. This is a human sewer.”

The lake in Harrow Lodge Park, which has Green Flag status, is the end of the Ravensbourne river.

Mr Thomas said pipes from neighbouring homes run alongside the river, leading to a main sewage pipe. When it rains a lot, sewers overflow and waste comes up from under the ground and manholes.

He suggested pipes could be too small or blocked by fat, estimating it could cost £10million to fix them.

Sandbags have been installed by Thames Water around one manhole to prevent dirty water coming up to ground level.

“It is clear they know the problem exists and not enough is being done to resolve it,” said Mr Thomas.

Resident Lorraine Moss, who joined Mr Thomas on his visit, said: “It’s a major concern because the lake is actually a human sewer and people take their kids to go and play there.”

Mr Thomas warned Thames Water could be breaking the law under the Environment Permitting Regulation of 2010.

But Thames Water said it did not plan to apply for a permit as the leak was not intentional, and it would be up to the Environment Agency to decide whether the law had been broken.

The agency said it had been told by Thames Water that aeration equipment had been installed to improve water quality and no sewage had reached the lake.

Cllr Melvin Wallace, cabinet member for culture and community engagement, said the council would assist Thames Water.