Jo Coburn reports from Crayford

The sewer involved carries all the waste from the nearby town of Dartford.

The Environment Agency says it is one of the worst spillages of its type and has wiped out vital fish stocks in the surrounding area.

A valve was tampered with

The vandals cut through a chain fence and closed the valves of a sewage pipe leading to a nearby treatment works.

This caused a build-up of pressure which blew open a manhole cover.

Workers had to overcome ammonia fumes to stop the flow of sewage, which is thought to have continued for three hours.

The pollution wiped out all life in a 200-metre stretch in the River Cray, killing fish including pike, roach and rudd.

Oxygen levels should read 40%

The sewage then cascaded over a weir and entered the Thames and the River Durrant where it is believed to have caused further damage.

Pollution experts from the Environment Agency are investigating the incident.

Samples taken from the river reveal levels of oxygen a tenth of what they should be.

Richard Maille of the Environment Agency gives details of the spill

Richard Maille, of the Environment Agency, said despite the loss of wildlife there was no great risk to the public.

"This part of the river isn't used for a drinking water supply so there's no need for concern with regards to any water that is going to be drunk by members of the public.

An Environment Agency investigation is underway

"The only risk would be if anyone actually came into contact in the immediate vicinity of where the incident actually happened and measures have been taken to get people away from the area."

He would not speculate on the possible long-term effects of the spillage, although said it would be weeks before natural processes in the water meant the river recovered in any way.

It is unclear how much longer it will be before fish return.

The pumping works is operated by Thames Water and campaign group Friends of the Earth, said the company was responsible for the leak.

Dr Michael Warhurst, FOE's industrial and pollution spokesman, said: "This is a large and damaging amount of sewage and it is going to take a long time for the river to recover."

The water company should have made sure the sewage valve was better secured, he added.

Security: Padlock and barbed wire protection

Thames Water says the leak happened because an intruder broke into the station and tampered with equipment.

Spokeswoman Helen Burton defended the padlock, barbed wire and metal fence surrounding the valves.

"It is an event that's happened. It isn't something we could really plan for and I think our security measures we've got in place at the moment are adequate."

The Environment Agency confirmed further action was likely.

Richard Maille said: "We have categorised it as a major incident because it had a high potential threat to the environment.

"So we'll be looking at prosecuting someone for this offence."