A water company was today slapped with a huge fine after "lamentable" failures at a "clapped out" treatment works led to raw sewage pouring into a stream for nearly half a year.

Thames Water has been hit with a £380,000 fine after numerous failures over a five-month period were reported at a Princes Risborough sewage treatment works - leading to traces of sewage being found up to 4km downstream and killing wildlife, Aylesbury Crown Court was told.

Thames Water engineers insisted there were no problems with the treatment works despite inspectors being able to smell sewage.

The company was fined after admitting to a breach of environmental regulations - prompting the firm's chief executive officer to make a pledge to the court to make significant improvements to the site.

In a scathing attack on the equipment and the team in charge of its upkeep, Judge Francis Sheridan said: “The truth is it was clapped out and this should have been picked up by management - if they are to operate for Thames Water under that title.

“They must invest in proper emergency equipment to enable their staff to deal with this kind of situation.

“It was entirely foreseeable that blockages would result in pollution to the stream. Nobody, in any form of position, could possibly have thought this site was compliant.

“Frankly they are unfit to be employed in a position of management if that's their view.”

The problem was reported to Thames Water by fisheries inspectors on February 19, 2013, and an engineer from the firm was sent to the site.

Judge Sheridan said: “They reported everything was fine. I find that incredible. It's an extraordinary failure.”

The bed of a stream in Horsenden was left covered in a layer of thick sludge as a result of the leak, killing most of the wildlife living in it.

Ammonia levels beyond the treatment plant were five times higher than they were upstream.

Dead animals were found 2.6km downstream from the treatment works and traces of sewage were discovered 4km away, Judge Sheridan said.

Excessive amounts of rainwater led to overflow tanks at the treatment works filling up - allowing sewage to flow out and into a stream, the court was told.

"Storm tanks" containing excess water were not emptied and cleaned as soon as possible, in line with regulations, a judge noted.

The court heard that Thames Water has 162 previous convictions for environmental offences, mostly for allowing effluent to be discharged into water.

Judge Sheridan fined the company £380,000 and ordered them to pay £23,092.62 in costs to the Environment Agency, which brought the prosecution.