The latest pollution of the River Crane has been blamed on fat blocking sewage pipes.
Sewage was released into the river, which runs from Hayes to Isleworth, last Wednesday (July 2), after the sewage network became blocked by fatty build-ups.
Thames Water said the latest release, at the junction of Whitton Brook and the River Crane, did not appear to have damaged wildlife or vegetation along the river as far as it or the Environment Agency could see.
It said the blockage was cleared early in the morning of last Thursday and it had used aeration equipment to increase depleted oxygen levels in the water, as well as carrying out clean-ups 'where necessary'.
Thames Water said it was inspecting and cleaning its sewers to prevent a repeat but said the incident highlighted the harm being caused by pouring fat down the drains.
In a statement published last Friday on the website of the Crane Valley Partnership, the group set up to restore and maintain the river, the company said: "Hounslow is one of our worst three boroughs (out of more than 100) for blockages caused by people putting wet wipes, fat etc down drains.
"We have a major campaign being planned to highlight this issue and would welcome support from CVP members in due course. We will continue to work with the Environment Agency until the river is completely back to normal."