Concrete is blocking a sewer in Hounslow which serves hundreds of homes, causing water to back up into some properties.
The blockage is in a metre-wide pipe serving homes on the Frampton Road estate, beside Hounslow Heath, where Thames Water said several residents had complained on Wednesday (May 11) morning of water draining slowly and toilets not flushing.
The utility firm has provided a tank to pump away sewage and prevent it backing up into people's homes while it assesses the extent of the damage.
A spokeswoman for the company said it might be possible to blast the concrete away, but if not the affected section would have to be cut out which could take around three weeks.
If that was necessary, she said temporary overground pipes would have to be laid while the work was carried out.
'Concrete blockages a common and frustrating problem'
She told getwestlondon it was far from the first time concrete had blocked a sewer, with developers usually responsible for such damage.
"Third parties putting concrete in our sewers is a common and frustrating problem for us and our customers," she said.
"The concrete sets hard and blocks the pipes meaning they have to be dug out and replaced with new ones.
"This can often take a long time and cause traffic disruption. In this case, to avoid sewage flooding properties and the environment, we're also using tankers to suck up the waste water that would normally flow through the blocked pipe.
"It's unclear at this stage how far along the pipe the concrete went so we don’t know how much longer our work will take. We will however do everything we can to get the job done as quickly as possible."
Hounslow was recently shamed by Thames Water as a hot spot for "fatbergs" , which are blockages of congealed fat and wet wipes which have been poured down drains or flushed down the toilet.