The deadly chemical cyanide and a quantity of raw sewage have leaked into a 30-mile stretch of the River Trent in Staffordshire.
Thousands of fish have died and people are being warned to stay away from the river while the incident is controlled.
The Environment Agency said pollution on the stretch between Stoke-on-Trent and Yoxall posed a health risk.
A spokeswoman said the warning applied to the public, farmers, anglers, boaters and pet owners.
Alan Walters, from the Environment Agency, told BBC News the incident had had "a huge impact".
He said the agency was attempting to use hydrogen peroxide to offset the worst effects of the spillage.
Andrew Marsh, from Severn Trent Water, said a leak of cyanide, which had not been caused by the company, had knocked out a water treatment plant and this had led to a discharge of sewage.
A spokeswoman said the company did not get its water from the Trent and the water supply had not been affected "at all".
The Environment Agency said the pollution was expected to reach Burton upon Trent by late afternoon on Wednesday.
'Tracking' pollution
It is thought the risk to the public may have been reduced by then but officers are continuing to monitor the situation.
A spokeswoman said an investigation had begun into the cause but could not give further information into the source of the contamination for legal reasons.
The cyanide and untreated sewage were at levels to give "cause for concern, especially with regard to fish, wildlife and animals".
Thousands of fish have already died but it is important that farm animals and dogs are kept out of the water.
"We are tracking the pollution as it moves downstream," the spokeswoman added.
A spokeswoman for the RSPCA urged members of the public not to go near any animals in distress but instead to report any sightings to the charity.
She said: "For those people with livestock we would remind them to get them away from the area as safely as possible."
From BBC News
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