Sewage spill kills fish in River Crane near Heathrow
About 3,000 fish have died after raw sewage spilled into the River Crane in west London, conservationists said.
Thames Water said a jammed valve in a major sewer serving Heathrow Airport led to the sewage spilling from Sunday afternoon until early on Monday.
The firm said it was faced with the "unpalatable" option of allowing excess sewage to back-up to the airport or release it into the river near Feltham.
The Environment Agency has begun an inquiry into the "major incident".
Fish killed in the spill in the tributary of the River Thames include pike, eel and perch, some of which were five or six years of age.
About seven miles of the river has been affected and the agency said its officers have been working round-the-clock since the spill and were monitoring levels of dissolved oxygen and ammonia in the river.
The water company said the two-metre, six-tonne cast-iron valve, located at Cranford Bridge, became jammed during a routine maintenance on Saturday afternoon. Special hydraulic lifting equipment was used to force it open on Monday morning.
A statement from Thames Water said it was able to take away a lot of the backed-up sewage in tankers, but not all.
"Faced with the unpalatable choice of letting the remaining sewage back up into the airport or spill to the River Crane, we were forced to opt for the latter, resulting in sewage entering the river and damaging wildlife, which we deeply regret," it added.
There will be a stronger than normal smell around the area near Mogden sewage treatment plant when normal flow resumes, the firm said.
The Environment Agency's area manager, Julia Simpson, said: "We have launched a full investigation into this major incident which has caused the death of approximately 3,000 fish.
"Our officers have been out all day, monitoring levels of vital nutrients in the river and are helping and overseeing the clean up operation.
"We are still advising people to keep themselves and their pets out of the water until the incident is resolved."