Raw sewage has been found in a brook that runs through two of the borough's parks.
Concerns were raised after a maintenance staff found the waste in Roxbourne Brook, which winds through Rayners Lane and South Harrow.
The stream runs through Newton Park West and Newton Farm Park in the south of the borough and behind homes in Leaminton Crescent and Stuart Avenue.
Che Keogh, who regularly walks his dog in Newton Park, said: “The brook is just disgusting and I would never let my dog near it.
“It’s just been left for years and something needs to be done to clear it up so it’s safe for children and dogs.
“A few years ago my old dog went in the stream and as soon as it got home he was ill. It’s not safe for anyone.”
Labour Cllr Bob Currie said: “The state of the brook is disgusting. All the relevant authorities need to resolve this, and ensure it doesn’t happen again, as soon as possible. Local residents deserve better.”
A Harrow Borough Council spokesman said: “A council officer will visit as a matter of urgency to ensure this site is cleaned up.
“One possibility is that a domestic sewage pipe may have been incorrectly connected by a local resident or business but we are carrying out a range of checks.”
The spokesman said the council would work with Thames Water and the Environment Agency to investigate the cause of the problem.
An Environment Agency spokesman said: “We are aware small amounts of sewage being deposited in the Roxbourne Brook in South Harrow.
“Due to the nature of the pollution, we have conducted a campaign to inform local residents of their responsibilities around the watercourse and provided nearby houses with literature informing them of the situation.
“As there is evidence to suggest the issue may be linked to over-occupancy and illegal tenancy in the area, we have been supporting our partners at Harrow Council’s environmental health team as they lead the investigation.”