Plumbing connections that were not properly made have caused businesses and properties in Devon to leak sewage into water systems, the Environment Agency (EA) has said.
EA said it has investigated more than 200 incidents of sewage flowing into surface drains rather than sewers in the Plym Estuary area since January.
The agency said that while most cases are probably accidental, particularly where there have been changes or extensions to properties, it might be due to ‘unscrupulous tradesmen’.
"It is quite a serious problem, as it is having an impact on water quality," Mike Ingman from EA told BBC News. "In some cases unscrupulous tradesmen could be connecting in where it is cheaper or easier for them to do so."
EA said problems could range from washing machines that have been wrongly plumbed in, to commercial businesses or schools discharging sewage into streams and rivers.
A pub-restaurant in Plympton was found to be discharging fatty food waste into a storm drain, blocking the drain which was used to alleviate flooding.
EA, South West Water and Plymouth City Council have been investigating the problem as part of the Plymouth Urban Diffuse Pollution Project by using cameras, fluorescent dye and echo testing at sewage outfalls and under manhole covers. The project will soon start investigating the Tamar Estuary.