Southern Water has been fined £330,000 after raw sewage escaped into a stream for up to 20 hours killing 2,000 fish after the water company failed to respond to an alarm set off by failing equipment.

The water company was sentenced at Southampton Magistrates’ Court after pleading guilty to a breach of environmental regulations regarding the pollution and operation of Little Bull pumping station in the case brought by the Environment Agency.

The leak on July 21 2019 spilled untreated effluent into Shawford Lake Stream in Waltham Chase, Hampshire.

As well as polluting the stream, the sewage went across fields and into the site of the YMCA Fairthorne Manor activity centre which was compensated for having to cancel 1,000 bookings with no water sports taking place for 10 days.

An Environment Agency spokesman explained that the pumping station had been wrongly programmed.

He said: “This led to a pump failing. When a second one wouldn’t start, sewage and other hazardous substances were diverted up through two manholes, across fields and into Shawford Lake Stream, leading to the popular YMCA Fairthorne Manor.

“In the days after the incident in July 2019, investigators from the Environment Agency found pools of dirty water and polluted matter and vegetation in local fields.

“The stream was cloudy as pollution spread across nearly 3km. Ammonia levels in the water were 25 times the legal limit.

“Scores of brown trout and other dead fish continued to be discovered. Tens became hundreds as the scale of the pollution emerged.”

He added: “Investigators believe the illegal flow of contaminated matter continued over public land and the stream for between five and 20 hours.

“The number of fish killed grew as the investigation went on. Brown trout, bullheads and sticklebacks all found lifeless in the water – 1,954 in all. Investigators saw no live fish in parts of the stream, only dead ones.”

Dawn Theaker, environment manager in Hampshire for the Environment Agency, said: “Yet again, we have a water company failing to properly respond to alarms when things go wrong at facilities they operate, allowing sewage to flow uncontrolled into fields and a stream.

“Any pollution is unacceptable, but this one happened close to a Site of Special Scientific Interest and other designations meant to provide greater protection for nature.”

Water minister Robbie Moore MP, from the Department for Environment, Food and Rural Affairs, said: “Southern Water have rightly been punished today for damage to our natural environment and it’s just the latest example of how polluters are being held to account.

“Today’s fine will be paid into our Water Restoration Fund, which will support further work we are already doing to clean up our waterways.”

Richard Manning, Southern Water’s general counsel and company secretary, said: “We are very sorry that this unacceptable historical incident in 2019 led to environmental damage.

“As soon as we became aware of this event, we took action to reduce its impact on the local area, and have since co-operated fully with the Environment Agency’s investigation, pleading guilty at the first opportunity.

“In acknowledgement of our role, we have already compensated the YMCA and set up a £140,000 grant scheme with the Groundwork South Trust to aid habitat improvement.

“Learning from this incident also led to a comprehensive review of our more than 3,000 unmanned pumping stations to ensure those at highest risk were fit for purpose, leading to a rolling programme of improvements to equipment and monitoring technology which has already cost more than £13 million.

“Almost five years on from this incident, we now have a new leadership team and shareholders, and are continuing to deliver our landmark Turnaround Plan at pace – ensuring improvements are achieved across the board, including tackling pollutions which have been slashed from 430 in 2019 to 358 last year and a further dramatic drop is due to be announced shortly.”