Warning over River Aire diesel spill
Thousands of gallons of diesel have leaked into a seven-mile (12km) stretch of the River Aire in Leeds.
A number of contractors are on the scene attempting to absorb the oil.
The Environment Agency has appealed to businesses in the Beeston and Holbeck areas to check their equipment after fears it may have leaked from an engine.
Members of the public have also been asked to be vigilant and to ensure their dogs do not enter the water.
Team leader for the Environment Agency Mark West said: "We are appealing to businesses in the area, especially those that have large quantities of diesel or kerosene stored either above or below ground.
"Someone somewhere has lost and could be continuing to lose very large quantities of oil.
"Not only is this causing serious pollution they are losing potentially large amounts of money as the fuel drains away."
A spokesman for the agency said that although no fish had been found, it was concerned about the effect on wildlife in the local area, including salmon.