Thames Water has been fined £125,000 at Croydon Crown Court for causing the death of two tonnes of fish in the River Wandle.

The dead animals had to be sifted from the river after the company spilled industrial-strength bleach into the waterway.

The water supplier faced a severe fine after pleading guilty to inadvertently releasing 1,600 litres of sodium hypochlorite into the river in September 2007.

A Thames Water lawyer was unable to convince magistrates to impose their maximum penalty of £20,000 in December and the case was taken to the Crown Court for sentencing.

In addition to the hefty fine, the company was ordered to pay £21,335 in clean-up and investigation costs.

A mechanical fault at Beddington Sewage Works allowed the chemical, similar to that used in swimming pools, to flow into the Wandle.

But the court heard that employees, who were cleaning the sewage works at the time, did not think the spill serious enough to alert the Environment Agency.

Several miles of the river had turned white within hours. Dead fish were seen floating in the river with eels bleeding from the gills.

Two police officers sent to investigate found the smell of bleach so strong that they felt nauseous.

Since the incident Thames Water has pledged £500,000 to the Wandle Trust and river restoration.

In sentencing Thames Water the Court took into account the company’s early guilty plea and its work with the Environment Agency on the River Wandle since the incident.

David Owens, Chief Executive of Thames Water, said: "We take full responsibility for this deeply upsetting incident, which resulted in serious environmental damage to the River Wandle, including the deaths of large numbers of fish.

"Immediately after the event we apologised unreservedly to the local community and backed that up by committing £500,000 to restore and improve the river over five years.

"That work, with the Wandle Trust, the National Trust, the Environment Agency and the Anglers' Conservation Association, has already started and we are pleased that the judge recognised this.

“We have learnt important lessons from this serious incident and have since strengthened our procedures at all our sewage treatment works.

"Any pollution incident is one too many, but the company's record has greatly improved over the last two years under new ownership and we are determined to do even better."