A water company has been fined more than half a million pounds after raw sewage was discharged into a river for 23 hours killing 5,000 fish, the Environment Agency said.

Anglian Water pleaded guilty to a breach of permit and was ordered to pay a fine of £510,000, costs of £50,000 and a victim surcharge of £170 at Peterborough Magistrates’ Court on Thursday.

About six million litres of raw sewage was pumped into the Great Ouse at Brackley, Northamptonshire, during the incident.

The discharge, from the emergency overflow at the pumping station, started just before 6pm on May 24 2017 and continued until about 5pm the next day.

Anglian Water said this was “deeply distressing”, adding: “We take our duty of care to the environment incredibly seriously and deeply regret any negative impact when things go wrong.”

Electrical faults caused the pumps to stop and then a failure of the early warning alarm system meant the discharge went unnoticed.

The pollution was found to have stretched 12 kilometres down river, the Environment Agency said.

Fish including brown trout, chub and pike were killed, as well as smaller species such as bullhead, dace, stone loach, minnow, gudgeon and 79 brook lampreys. Dead signal crayfish were also spotted.

Environment manager at the Environment Agency Andrew Raine said: “The environmental impact of this pollution was substantial, resulting in a large-scale fish kill and affecting 12km of the upper River Great Ouse.

“Polluters should always be held to account and, as much as our resources allow, we will always investigate significant pollution incidents and bring those responsible before the courts.

“We are grateful that the level of fine acknowledges the damage to the river ecosystems that this sewage spill from Anglian Water has caused.”

Sir James Bevan, chief executive of the Environment Agency, added: “We welcome this sentence.

“Serious pollution is a serious crime.

“The Environment Agency will pursue any water company that fails to uphold the law or protect nature, and will continue to press for the strongest possible penalties for those which do not.”

An Anglian Water spokesperson said: “We work tirelessly to protect and enhance the environment, and find it deeply distressing when incidents like this occur.

“We know there’s no room for complacency, and we’re absolutely determined to improve further and progress towards achieving our zero pollutions goal.”