A large maritime paint manufacturer was fined £650,000 when a highly dangerous prohibited chemical spilled out of a holding tank and into a "pristine" river in southwest England.

A judge decided that International Paint Ltd "utterly failed" to regulate a material known as TBT that it had stockpiled at its decommissioned plant on the banks of the Yealm river in Devon.

An expert who analyzed water in the river, a special scientific interest location known for its oysters and cockles, discovered that one sample near the factory contained 80,000 times the allowable level of TBT.

TBT was used in anti-fouling paints for ships to prevent the growth of barnacles and algae until it was prohibited worldwide in the 2000s due to its toxicity to invertebrates. Mollusks are particularly vulnerable to it.

It is worth noting that International Paint was fined £650,000 and ordered to pay £145,00 in costs. The company has agreed to pay for remediation costs, which are expected to reach at least £500,000.

Recorder Simon Levene sentenced the company, which is controlled by the multinational AkzoNobel, noting it stopped using TBT in 2002 and should have cleaned out the tank years ago. He described the discharge as "suspicious" because it occurred after the facility was placed up for sale.

"Though I don’t believe anybody directed the TBT should be washed out of the tanks, it is suspicious that the TBT was only discharged when a potential purchaser for whom the presence of TBT in the tanks was a serious problem came along," Levene indicated.

He added, "I am quite satisfied that the defendant, having closed its eyes for years to the problem, operated a reckless system in which it utterly failed to control the management of TBT and other chemicals. I’m satisfied that [a caretaker] emptied the TBT into the estuary and that is something that should never have been allowed to happen."

The judge, sitting at Plymouth Crown Court, also expressed alarm about the "astronomic" quantities of mercury found in the river. The Food Standards Agency will look into whether this could have entered the human food chain through shellfish.

International Paint Ltd had disputed two counts pertaining to the discharge of hazardous waste from a tank on the quay at its Newton Ferrers paint testing plant, but a jury found it guilty.

According to the court, the Environment Agency initiated an investigation after the company attempted to sell the premises and suspected pollution was discovered.

Since 1928, International Paint Ltd had operated a testing station on the Yealm at Newton Creek in Newton Ferrers. TBT was phased out by the corporation in the early 2000s, and the location was dismantled in 2013. However, the TBT and other chemicals were not removed, and the caretaker dumped the contents into the river in 2016. According to the judge, the river had been a "pristine environment" before the tragedy.

Read next: UK water companies discharge sewage into water, protected by monopoly