Canal water in the West Midlands is being tested for sodium cyanide after a spillage of toxic chemicals.
Walsall Council said the Environment Agency told it about a spillage that went directly into the canal at Pleck on Monday.
A major incident has been declared, involving police, the fire service, the Canal and Rivers Trust and Severn Trent Water.
Exposure to sodium cyanide can cause headaches, nausea, dizziness, confusion, changes in heart rate and loss of consciousness, Public Health England says on its website.
Ingesting cyanide salts, which can dissolve in water, releases cyanide into the body, the agency adds.
In high enough concentrations it can be fatal.
There is potentially a “serious risk to health” if people or their pets are exposed to the chemical by touching the water, Walsall Council said.
Anyone who has taken fish from the canal is advised not to eat it.
Members of the public are being asked to avoid an “extended area of the canal and towpaths” from the “Walsall lock flight to the lock flights at Rushall/Ryders Green and Perry Barr”.
The Environment Agency is testing the water for “sodium cyanide and other chemicals”, the council said.
It is hoping to “reduce the impacted area as soon as possible”.
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Councillor Garry Perry, the leader of Walsall Council, said: “Our priority is the safety of our residents.
“We are working closely with our partners to manage this situation which has been declared a major incident.
“For your own safety, please avoid this area of the canal and its towpaths.”
Sodium cyanide is used in industry for metal cleaning, plating, extraction and photography.
