A waste company has been banned from discharging industrial effluent into sewers by officials investigating a cyanide leak in the River Trent.
Tests showed the presence of cyanide at the sewer discharge point used by Red Industries Ltd, of Burslem, Stoke-on-Trent, the Environment Agency said.
Tens of thousands of fish died and people were warned away from a 30-mile stretch of the Trent on Tuesday.
Red Industries confirmed it was working closely with investigators.
The agency said as part of its investigation it had served a "suspension notice" to prevent Red Industries from discharging any industrial effluent to the sewer system.
Investigators have been at the company and in a statement the agency said the issuing of the notice followed its "swift action" in its continuing investigation into the source of the cyanide pollution.
In a statement Red Industries said it had "voluntarily stopped" discharging industrial effluent before the suspension notice.
The firm added: "We have always worked closely with the Environment Agency and will continue to assist them fully in their investigation."
Meanwhile cyanide levels in the river between Stoke-on-Trent and Yoxall have "greatly reduced".
David Hudson, environment manager at the Environment Agency, said: "The pollution has killed thousands of fish on the River Trent and caused a great deal of concern for many people.
"This sort of incident is unacceptable.
"Our investigation is continuing to ensure we can take firm action against those responsible."
On Thursday the Environment Agency was still advising people and animals to keep out of the 30-mile stretch of river as a precaution.
Officers began pumping oxygen into the water on Tuesday night to reduce the pollution levels.