Levels of cyanide and untreated sewage in 30 miles of one of country’s major rivers were falling today as experts continued their investigation into how the water became polluted.

Environment Agency officers were out overnight pumping oxygen into the River Trent in Staffordshire after the “serious pollution incident”, which has already killed thousands of fish.

The agency said it had not received reports of any humans or domestic pets being affected by the contaminated water, but would seek to prosecute following an investigation that is under way.

Reissuing a warning for people and animals to keep out of the river between Stoke-on-Trent and Yoxall, a spokeswoman said: “We have had environment officers out all night and today pumping oxygen into the River Trent, which has been polluted with cyanide between Stoke-on-Trent and Yoxall.

“Samples taken throughout the night show pollution levels have already dropped. We expect further dilution today as the Trent meets the River Tame.

“We are still asking boaters, anglers, farmers and dog-walkers to stay away from the affected stretch of river as a precaution.”

The spokeswoman said the contamination was being treated as a “serious pollution incident” and could lead to prosecution.

She added: “It is very important we know where these substances came from so we are focused on that.”

No precise timeframe has been put forward but the Environment Agency said it hoped for a “swift resolution” and was monitoring the contamination as it moved downstream.

Officers were working with the Drinking Water Inspectorate and the Food Standards Agency and members of the public were urged not to extract water from the river.

A spokeswoman for the Environment Agency’s Midlands regional office said river levels had been “fairly low” and the risk of contaminated water overflowing was minimal.

The RSPCA urged members of the public not to go near any animals in distress but instead to report any sightings to the charity.

A spokeswoman said: “What we are trying to advise members of the public to do is, if they see any animals in distress, not to go near them but to report them to our Cruelty and Advice line, like details of where, what and so on.

“We will probably be having a team go to the area and we will be getting involved with the Environment Agency and other agencies.

“We expect there will be quite a big clean-up operation involved.

“For those people with livestock we would remind them to get them away from the

area as safely as possible.”

Anyone who spots animals in difficulty is asked to call the RSPCA’s Cruelty and Advice line on 0300 1234 999.

The Environment Agency manages fisheries, flood defence, navigation, recreation and nature conservation. In Scotland, the River Purification Boards control river pollution.

The Water Resources Act 1991 allows the Environment Agency to police the use of water in England and Wales.

Last month in a separate incident, water company Severn Trent Water was fined almost £7,000 for allowing pollution to enter the River Trent 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 River Trent.