Ewell Court Pond seems to be polluted again, with an oily sludge spotted on the surface of the water.

But this time, the effluent is believed to be from flytippers rather than dodgy plumbing.

Four years ago, raw sewage from four toilets and pollution from 51 appliances were found to be causing the nasty problem.

Thames Water and the Environment Agency painstakingly traced the source of pollution over three years, using orange dyes and CCTV, and in 2014 announced the problem was “sorted”.

A Guardian reader told us Thames Water was there earlier in the week investigating the new pollution.

He said: “It appears to have moved down the stream as my dog came out covered in diesel, or what I think is diesel.

“I believe some of the birds were taken away for cleaning.”

A spokeswoman for Thames Water said: "It's not the same issue, we believe somebody is flytipping, it's not something we can chase."

In 2002 the council forked out £100,000 to clean the lake, not realising the problem was caused by wrongly connected drains.

The extent of the problem only became apparent once the well-intended clean-up was thwarted.

A spokesman for Epsom and Ewell Council said: "The Environment Agency have inspected the lake and have taken samples but have yet to trace the source of the contamination.

"Council rangers are monitoring the wildlife in and around the lake."