Thames Water is likely to face prosecution over the devastating Wandle bleach spill which led to the death of 1.4 tons of fish.

The water company admitted responsibility for the spill of sodium hypochlorite - or household bleach - from the Beddington sewage works on September 17, which poisoned at least 2,000 fish.

The Environment Agency (EA) is still analysing water samples, but hopes to prosecute and recoup the cost of a cleanup operation.

Mike Denbigh, an environment manager from the EA, said: "This is a category one event, based on the level of fish killed. We have a policy with incidents like this to gather evidence sufficient to undertake a prosecution."

Denis Horsman, Thames Water's event manager for the incident, said the bleach came from the Beddington sewage works and was spilled during cleaning.

The bleach was supposed to be poured into a water tank, but was allowed to pour out into an overflow which went straight into the river.

Mr Horsman apologised and said Thames Water would do whatever it took to put the mistake right.

"We are devastated by it," he said. "We have contributed a lot to improving the Wandle over recent years. We are carrying out an investigation and any learning from this event will be taken to all our plants."

Fisheries experts gathered at the Wandle near Ravensbury Park on Friday for an electrofishing operation - a process of stunning fish with an electric current in order to catch and count them, before releasing them back into the river.

The EA has spent the last decade reviving the Wandle's ecosystem.

They found 32 fish and eels in the 100m stretch of river. They had probably survived the pollution by either burying themselves in the riverbed or hiding in clean pockets of water at the river's edge, fisheries and biodiversity expert John Sutton said.

Fisherman Jed Edge was at the riverside to watch the work. He said: "It is devastating. This was one of the best places rivers to fish in the south of England and I love it here."