Thames Water has aplogised after a chemical spill from a sewage treatment works caused thousands of fish to perish in the river Wandle.

Barbel, roach, chub and eels were among the many species that perished when cleaning solvents from the company's Beddington plant discoloured water and caused a strong bleach-like smell to rise from a 5k stretch of the river between Beddington and Merton on Monday.

Conservationists say the spillage has caused years' worth of damage.

Fishery officer at Morden Hall Park Jed Edge said: "Dead fish were floating to the surface. The water was bubbling and people said their eyes were smarting and there was a terrible smell of ammonia.

"It's absolutely devastating - the worst case of pollution I've seen in years."

Bob Collington, director of wastewater services at Thames Water, said the company would do as much as possible to bring the river back to full health.

"A thorough investigation is being conducted and we are taking immediate steps to ensure this sort of event cannot happen again.

"Our top priority now is to work closely with the Environment Agency on cleaning up the affected areas of the river, and we will do everything we can to restore the river's ecosystem, including replacing lost fish and affected habitats."

The Environment Agency has been at the scene collecting evidence about the outbreak which killed at least 2,000 fish.

A spokesman said: "This is a serious incident.

"Our officers are continuing to assess conditions along the river, but it will take some time for us to determine the full environmental impact."

This is the second time pollution has leaked into the Wandle in recent weeks.

After the first incident last month, the Environment Agency said it was investigating the cause.