A broken pipe at Didcot Sewage Works caused chemical contamination of a local stream, killing an estimated 1,500 fish and water shrimps.

Sixty litres of strong bleach leaked into the Moor Ditch near the Southmead Industrial Park in what was described to Wantage magistrates as "an unfortunate accident".

The incident came to light on August 24 last year when an employee of a neighbouring company smelt chlorine and noticed fish dying in the stream, a tributary of the River Thames, the court heard.

In the prosecution brought by the Environment Agency on Wednesday, Thames Water Utilities pleaded guilty to causing polluting matter to enter the Moor Ditch under Section 85 of the Water Resources Act 1991. It was fined £20,000 and ordered to pay £1,564 costs.

During the investigation, Thames Water admitted there had been a spillage of sodium hypochlorite, a strong bleaching agent which releases chlorine when mixed with water.

The spillage was due to a broken pipe in the sewage treatment system, which allowed the chemical to discharge into the ditch via sand filters in the final treatment plant.

The hazardous chemical is used at the works as part of the final stages of sewage treatment to prevent algae growth on the filters. It has to be used in very small doses because it is highly poisonous to aquatic life.

A water sample showed levels of chlorine 200 times the proposed environmental quality standard. Fish restocking of the Moor Ditch has now taken place and the watercourse is expected to recover.