Weetabix has been fined £140,000 for polluting a river with thousands of litres of diesel fuel and putting at risk fish and plant life.

The Environment Agency brought a prosecution against the cereal maker for polluting the River Ise, which is close to the company’s plant in the town of Burton Latimer, near Kettering, Northamptonshire. The clean-up operation cost Weetabix £500,000.

The firm, which uses locally grown wheat, has been manufacturing the breakfast cereal in the area since 1932. Weetabix was bought by the US company Post Holdings in 2017 in a deal worth £1.4bn. The company has an annual turnover of £300m.

Judge Rupert Mayo, recorder for the borough of Northampton, issued the fine of £139,694.72 in Northampton crown court on Thursday after the cereal company pleaded guilty.

Sailesh Mehta, prosecuting on behalf of the Environment Agency, referred to “corporate amnesia” by Weetabix in its failures to carry out appropriate checks on pipes and valves in storage facilities on the site.

Mayo said in his ruling: “My considered conclusion is that this corporate amnesia amounted to a negligent failure … Weetabix failed to keep records of what was on their site. Their plea recognises this failure.”

The pollution happened on 10 November 2016 when 23,000 litres of diesel road fuel leaked from tanks contained in a bund on the Weetabix site and entered the river.

Wet weather combined with the way the river was flowing at the time limited the amount of damage to fish and plant life.

The judge said: “Had the diesel permeated the river in drier conditions there could well have been a significant adverse effect on animal health or flora.”

The leak occurred because of a problem with secondary valves which controlled the flow of the diesel fuel from five storage tanks overground.

There is a disused fuel storage facility onsite which was decommissioned approximately 20 years ago. In 2007 the Environment Agency sent Weetabix notices advising them to remove the pipework from the decommissioned tank but the company did not do this. When the valves in the fuel tanks overground were left open the fuel drained through these pipes into the river.

In his ruling Mayo said Weetabix should have had proper records of the valves and underground pipes in the former storage facility but the company’s risk assessment did not contain information about these secondary valves.

The judge accepted that before the incident Weetabix had a near spotless record on environmental compliance, employing highly qualified people to monitor health, safety and environmental factors. Jokes were made in court about the company not being a “cereal offender”.

A spokesperson for Weetabix Food Company said: “We respect the court’s decision and regret the circumstances of the incident. We have worked closely with the Environment Agency during their investigations, and our plea reflects how seriously we take our responsibility to the local environment.

“We have operated safely and securely from our site in Burton Latimer since 1932 and are proud of our local record which includes promoting sustainable farming through our Wheat Grower’s Group.”