Neighbours of the Mogden sewage works in Isleworth, Hounslow, have kicked up a stink amid a flurry of complaints about a sludge spill earlier this month.
A spokeswoman for owners Thames Water confirmed a safety overflow at the pasteurisation plant on Saturday, January 10, caused a minor sludge spill into a drain.
"This is a normal safety process for this piece of equipment and we carried out a clean up of the area immediately," she said. "We would like to apologise to any residents who may have experienced any odour as a result of this."
But Steve Taylor, secretary of Mogden Residents Action Group, said the smell lingered for much longer.
"Residents are quite astounded and appalled that Thames Water is apologising for 'any increase in odour that residents may have experienced', when it was quite obvious all around the site that the spill had a major environmental impact on both Hounslow and Richmond boroughs," he said.
MRAG has now forwarded 40 complaints about the stink to owners Thames Water.