Public health warnings were issued as the polluted water made its way downstream from Stoke-on-Trent towards the east Midlands and an investigation was launched into the incident, which killed thousands of fish and threatened other wildlife and pets.
The Environment Agency said people and animals should stay out of the river between Stoke and Yoxall. The Food Standards Agency said people should temporarily stop extracting water privately from the river.
The Drinking Water Inspectorate said public water supplies were not affected because there were no public abstraction points on the affected stretch.
The Environment Agency said the cyanide was thought to be from an illegal discharge which had affected bacterial treatment of sewage at Severn Trent Water's Strongford works at Barlaston, near Stoke.
A spokeswoman said that effluent released into the river was now at "acceptable" levels. Oxygen had been pumped into the river to prevent further fish deaths. Investigations were under way to identify the source of the cyanide.
David Lowe, the agency's environment manager, said: "The incident is under control - river water quality is improving. Levels of pollutant in the water have fallen significantly, but we continue to monitor the situation closely.
"As a precaution, people and animals should stay out of the water until further notice."
A statement from the water company said it had been dealing with "an illegal substance" in the sewers since the start of the week.
"This chemical is completely soluble and as a result it is impossible to prevent it entering and passing through the sewage treatment works.
"The effect this type of chemical has on the works is devastating, both as it passes through the works and pollutes the watercourse and for the company in terms of maintaining the normal sewage treatment operation."
Simon Cocks, waste water services director for Severn Trent Water, said: "I can confirm that our company is not linked to the disposal of the chemicals.
"Engineers at our Strongford works have been working day and night to get the works back up and running. We are deeply concerned about the impact this chemical pollution has had on our sewage treatment system and the community in which we operate. We are working hard alongside the Environment Agency to support their investigation and minimise the impact of this incident on the environment."
In Yoxall, the river was sludge coloured and a strong chemical smell wafted across the countryside.
Lowe said the fish kill had affected 20 miles of the river. Anglers had first spotted fish gasping for air "like canaries in a mine" on Monday, he said.
The levels of cyanide were less than one part per million but "aquatic life is very sensitive to poison," he said.
The cyanide had killed the bacteria used at the treatment works, and a combination of ammonia, from the sewage, and cyanide had killed the fish.
"Teams of people are working in Stoke-on-Trent to try to identify where the cyanide came into the sewerage system and why it happed. We hope to gather enough evidence to bring a case to court."
It was likely to have emanated from a metal-type industry.
David Matthews, a birdwatcher and environmentalist said: "It is really bad, absolutely appalling. This is bound to affect the birds as well as they feed on insects and peck weeds. Everything is going to be affected, not just the fish."
Last month, in a separate incident, Severn Trent was fined almost £7,000 after a prosecution brought by the Environment Agency.
The company was fined £6,700 and ordered to pay costs of £2,777.80 at Stafford magistrates court after pleading guilty to causing sewage pollution to enter the Trent.
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