Thames Water has been handed a £2m fine for a “reckless failure” that led to the polluting of a tributary of the Thames with raw sewage.

The £2bn-turnover utilities firm admitted breaching environmental regulations in relation to untreated sewage flowing into a brook near Milton-under-Wychwood, near Chipping Norton in the Cotswolds, in August 2015.

About 150 bullhead fish along a 50m stretch of the Idbury Brook were killed by the sewage, which the Environment Agency estimates flowed into the water for at least 24 hours. The court heard Thames Water ignored more than 1,000 alarms from the Bruern Road sewage pumping station in west Oxfordshire in the six weeks leading up to the incident.

But Kerim Fuad QC, defending, said Thames Water operated more than 4,700 pumping stations and there had not been an incident at the Milton-under-Wychwood site since 1993. He told Oxford crown court on Friday the company had invested £40m in improving waste water sites and had shown remorse and deep regret for the spill.

He added: “Any loss of fish or damage to the environment short or long term is extremely regrettable but the cause of this unfortunate event in many respects remains unknown.”

But Judge Peter Ross said the number of pumping stations run by Thames Water was irrelevant. Handing Thames Water the £2m fine, he said the risk of pollution at each site needed to be managed regardless of how many there were. He also ordered the firm to pay nearly £80,000 in costs.

But he said if Thames Water donated £200,000 to three local environmental charities he would reduce the total fine by that amount.

The judge also said there was a failure by the organisation to integrate new technology with human judgment. He said this created a culture that led to the high number of alarms being interpreted as a problem with the alarm system and not the pump system.

Ross branded the incident a “reckless failure” of the organisation to put in place measures to prevent a spill. He added: “It’s that failure to integrate the technological advances with human assessment, analysis and response which lies at the heart of the deficiencies that were created in this case.

“In order to be operated effectively there had to be a proper integration of the technology and human resources, adequate training for the latter and a culture ensures that the exercise of judgment is still undertaken by human beings.”

Ross said more effective management and training and the establishment of a proper culture would have prevented the incident although he accepted improvements have since been made.

He said the case had similar failings to some of the firm’s previous convictions, including the record £20m fine imposed at Aylesburycrown court in 2017.

He added: “I am sure that more effective management, training and the establishment of a proper culture would have ensured this episode could not have happened. Had lessons really been learned and implemented from the events in relation to the two previous convictions then this would not have happened.

“It is a seriously aggravating feature in this case that the defendant allowed such management and cultural failings to continue to persist within the organisation.”

Thames Water admitted at a previous hearing to breaching Regulation 12(1) (b) and 38(1) (a) of the Environmental Permitting (England and Wales) Regulations 2016.