Sewage has been flowing into “Whitstable’s hidden river” and ending up in the sea, investigators have discovered.

Wastewater from toilets and washing machines has been found in the Gorrell Stream by teams from Southern Water, and they say the blame could lie with cowboy builders or “careless housing developers”.

The problem, which originates from an unknown number of plumbing misconnections, is said to be having a “considerable” impact on water quality in the town’s harbour.

Rob Butson, whose team at Southern Water is investigating the issue, said: “Gorrell Stream is mostly hidden from view, running underground.

“But somewhere between where it rises at Duncan Down and its outfall there’s a misconnection or even a group of misconnections.

“Solving this problem will have a real impact on water quality in the harbour.”

In a video produced by the Kent Stour Countryside Partnership called “Whitstable’s hidden river”, the group says it was once an open waterway running between Duncan Down and the harbour, but as the town grew, it was closed over and “imprisoned” beneath streets.

The misconnections have resulted in wastewater flowing through surface water drains – rather than foul sewers – and into the Gorrell Stream.

Officials from Southern Water say they will work with those responsible for the misconnections to ensure they fix them.

“Misconnections mean water from washing machines and toilets goes straight into the stream where ordinary rain run-off should go,” a spokesman added.

“With a toilet flushing as much as 13 litres each time every day, the impact is considerable.

“It’s impossible to say who’s at fault – builders or careless housing developers can misconnect and it can also be caused by moving washing machines outside into a garage to save space.”

The firm says it is monitoring the impact on water quality in Tankerton - the nearest bathing area to where the stream discharges.