By Jeremy Lovell. LONDON (Reuters) - British environment authorities mounted a major cleanup operation on Wednesday after flooding forced hundreds of thousands of tons of raw sewage into the River Thames running through central London.



The Environment Agency said 600,000 tons of untreated sewage overflowed into the river as London's Victorian-era sewers were overwhelmed by torrential rains that brought chaos to parts of the city on Tuesday night.

"The overflow has to go somewhere, and the Thames is the natural place," a spokeswoman told Reuters.

The agency said the discharge had killed thousands of fish in the Thames which was declared dead due to pollution in the 1960s but is now reputedly the cleanest urban river in Europe.

But environmentalists have long complained that the city's aging sewer system -- which carries both foul sewage and excess rainfall -- is in disrepair, outdated and increasingly unable to cope with both the rising population and climate change.

When severe storms hit the city -- as on Tuesday -- the rainwater combines with the raw sewage and discharges directly into the Thames to stop it ending on the streets of the city.

Robin Clarke, Director of Waste Water Operations at Thames Water, which is responsible for the sewers, said in a statement that the company "deeply regretted" the loss of fish and was working with the Environment Agency to respond.

"The scale of the fish loss last night was very rare -- a very unfortunate side effect of the exceptionally heavy rainfall. Despite events such as last night, the Thames remains one of the cleanest metropolitan rivers in the world," he said.

In a report last October the London Assembly's Public Services Committee said up to one quarter of the city's brickwork sewers -- many of which date from the 1840s -- were either leaking severely or on the verge of collapse.

The report said that even in moderate rainfall there were regular small sewage discharges into the river. These overflow discharges cause localized pollution problems which are usually treated by two barges known as Thames Bubblers which aerate the polluted water and help microbes digest the sewage.

However, in cases of major overflows more drastic treatment is needed. Thames Water has been told to pour hydrogen peroxide into the water to reoxegenate it.

However, the Environment Agency -- the government's environment watchdog -- said it could take several days to restore the river to prime condition once again.