Thames Water has been fined £60,000 for allowing raw sewage to pollute a stream in Cholsey, near Wallingford, killing 12,000 fish.

One of 12,000 fish which were killed

The firm was also ordered to pay £4,350 in costs at Oxford Crown Court on April 25 after the court heard it had ignored alarms warning that untreated sewage was overflowing into Cholsey Brook from its treatment plant.

The company admitted the offence at an earlier hearing at Didcot Magistrates' Court on April 13.

Experts have warned it could take five years before the brook fully recovers from the damage.

Under normal circumstances, treated sewage is discharged into Cholsey Brook and the River Thames. But on September 7, 2003, the treatment works' filter system - which was being replaced as part of a £25,000 improvement scheme - was blocked by "excessive rags".

This meant the sewage, which should have been treated through the normal process, was diverted to back-up tanks, which could not cope and quickly overflowed.

The court was told only 10 per cent of treatment plants were manned and staff were warned of any problems via alarms.

But Sailesh Mehta, prosecuting on behalf of the Environment Agency, said Thames Water staff failed to respond to five separate alarms, which showed the level of sewage in the tanks was high.

Jonathan Barmard, for Thames Water, said it was the first incident of its kind at the Cholsey works, which had been operating since the 1950s.

He said: "When one looks at the facts this was attributable to a human error - a mistake which was made that will not happen again."

He said the alarm system had since been revised.

Mr Barmard said the death of fish - which included sticklebacks, pike, roach and bullheads, which are a protected species - was always regrettable.

After the hearing, Rod Gould, senior enforcement officer for the Environment Agency, said: "This was entirely avoidable - you can have the most advanced computerised, remote sensing system in the world but you still need a sufficiently trained and capable human to interpret the readout."

John Gilbert, Thames Water regional operations manager, said the company was considering appealing the sentence.

He said: "We deeply regret any damage caused to the environment. We are disappointed with the level of the fine.

"We have spent £25,000 at the site improving the system."